diff --git a/README.md b/README.md
index 27383efbf55..49d1168d884 100644
--- a/README.md
+++ b/README.md
@@ -62,11 +62,11 @@ Other status codes that can occur are:
 The api supports the following actions:
 
 **/api/states - GET**<br>
-Returns a list of categories for which a state is available
+Returns a list of entity ids for which a state is available
 
 ```json
 {
-    "categories": [
+    "entity_ids": [
         "Paulus_Nexus_4", 
         "weather.sun", 
         "all_devices"
@@ -103,8 +103,8 @@ Returns a dict with as keys the domain and as value a list of published services
 }
 ```
 
-**/api/states/&lt;category>** - GET<br>
-Returns the current state from a category
+**/api/states/&lt;entity_id>** - GET<br>
+Returns the current state from an entity
 
 ```json
 {
@@ -112,14 +112,14 @@ Returns the current state from a category
         "next_rising": "07:04:15 29-10-2013", 
         "next_setting": "18:00:31 29-10-2013"
     }, 
-    "category": "weather.sun", 
+    "entity_id": "weather.sun", 
     "last_changed": "23:24:33 28-10-2013", 
     "state": "below_horizon"
 }
 ```
 
-**/api/states/&lt;category>** - POST<br>
-Updates the current state of a category. Returns status code 201 if successful with location header of updated resource and the new state in the body.<br>
+**/api/states/&lt;entity_id>** - POST<br>
+Updates the current state of an entity. Returns status code 201 if successful with location header of updated resource and the new state in the body.<br>
 parameter: new_state - string<br>
 optional parameter: attributes - JSON encoded object
 
@@ -129,7 +129,7 @@ optional parameter: attributes - JSON encoded object
         "next_rising": "07:04:15 29-10-2013", 
         "next_setting": "18:00:31 29-10-2013"
     }, 
-    "category": "weather.sun", 
+    "entity_id": "weather.sun", 
     "last_changed": "23:24:33 28-10-2013", 
     "state": "below_horizon"
 }
diff --git a/homeassistant/__init__.py b/homeassistant/__init__.py
index 2e61a12a5fa..706cc5037be 100644
--- a/homeassistant/__init__.py
+++ b/homeassistant/__init__.py
@@ -3,13 +3,13 @@ homeassistant
 ~~~~~~~~~~~~~
 
 Home Assistant is a Home Automation framework for observing the state
-of objects and react to changes.
+of entities and react to changes.
 """
 
 import time
 import logging
 import threading
-from collections import defaultdict, namedtuple
+from collections import namedtuple
 import datetime as dt
 
 import homeassistant.util as util
@@ -78,33 +78,33 @@ def _matcher(subject, pattern):
     return MATCH_ALL == pattern or subject in pattern
 
 
-def split_state_category(category):
-    """ Splits a state category into domain, object_id. """
-    return category.split(".", 1)
+def split_entity_id(entity_id):
+    """ Splits a state entity_id into domain, object_id. """
+    return entity_id.split(".", 1)
 
 
-def filter_categories(categories, domain_filter=None, strip_domain=False):
-    """ Filter a list of categories based on domain. Setting strip_domain
+def filter_entity_ids(entity_ids, domain_filter=None, strip_domain=False):
+    """ Filter a list of entities based on domain. Setting strip_domain
         will only return the object_ids. """
     return [
-        split_state_category(cat)[1] if strip_domain else cat
-        for cat in categories if
-        not domain_filter or cat.startswith(domain_filter)
+        split_entity_id(entity_id)[1] if strip_domain else entity_id
+        for entity_id in entity_ids if
+        not domain_filter or entity_id.startswith(domain_filter)
         ]
 
 
-def track_state_change(bus, category, action, from_state=None, to_state=None):
+def track_state_change(bus, entity_id, action, from_state=None, to_state=None):
     """ Helper method to track specific state changes. """
     from_state = _process_match_param(from_state)
     to_state = _process_match_param(to_state)
 
     def listener(event):
         """ State change listener that listens for specific state changes. """
-        if category == event.data['category'] and \
+        if entity_id == event.data['entity_id'] and \
                 _matcher(event.data['old_state'].state, from_state) and \
                 _matcher(event.data['new_state'].state, to_state):
 
-            action(event.data['category'],
+            action(event.data['entity_id'],
                    event.data['old_state'],
                    event.data['new_state'])
 
@@ -304,7 +304,11 @@ class State(object):
         else:
             self.last_changed = last_changed
 
-    def to_json_dict(self, category=None):
+    def copy(self):
+        """ Creates a copy of itself. """
+        return State(self.state, dict(self.attributes), self.last_changed)
+
+    def to_json_dict(self, entity_id=None):
         """ Converts State to a dict to be used within JSON.
         Ensures: state == State.from_json_dict(state.to_json_dict()) """
 
@@ -312,15 +316,11 @@ class State(object):
                      'attributes': self.attributes,
                      'last_changed': util.datetime_to_str(self.last_changed)}
 
-        if category:
-            json_dict['category'] = category
+        if entity_id:
+            json_dict['entity_id'] = entity_id
 
         return json_dict
 
-    def copy(self):
-        """ Creates a copy of itself. """
-        return State(self.state, dict(self.attributes), self.last_changed)
-
     @staticmethod
     def from_json_dict(json_dict):
         """ Static method to create a state from a dict.
@@ -345,75 +345,75 @@ class State(object):
 
 
 class StateMachine(object):
-    """ Helper class that tracks the state of different categories. """
+    """ Helper class that tracks the state of different entities. """
 
     def __init__(self, bus):
-        self.states = dict()
+        self.states = {}
         self.bus = bus
         self.lock = threading.Lock()
 
     @property
-    def categories(self):
-        """ List of categories which states are being tracked. """
+    def entity_ids(self):
+        """ List of entitie ids that are being tracked. """
         with self.lock:
             return self.states.keys()
 
-    def remove_category(self, category):
-        """ Removes a category from the state machine.
+    def remove_entity(self, entity_id):
+        """ Removes a entity from the state machine.
 
-        Returns boolean to indicate if a category was removed. """
+        Returns boolean to indicate if a entity was removed. """
         with self.lock:
             try:
-                del self.states[category]
+                del self.states[entity_id]
 
                 return True
 
             except KeyError:
-                # if category does not exist
+                # if entity does not exist
                 return False
 
-    def set_state(self, category, new_state, attributes=None):
-        """ Set the state of a category, add category if it does not exist.
+    def set_state(self, entity_id, new_state, attributes=None):
+        """ Set the state of an entity, add entity if it does not exist.
 
         Attributes is an optional dict to specify attributes of this state. """
 
         attributes = attributes or {}
 
         with self.lock:
-            # Add category if it does not exist
-            if category not in self.states:
-                self.states[category] = State(new_state, attributes)
+            # Add entity if it does not exist
+            if entity_id not in self.states:
+                self.states[entity_id] = State(new_state, attributes)
 
             # Change state and fire listeners
             else:
-                old_state = self.states[category]
+                old_state = self.states[entity_id]
 
                 if old_state.state != new_state or \
                    old_state.attributes != attributes:
 
-                    self.states[category] = State(new_state, attributes)
+                    self.states[entity_id] = State(new_state, attributes)
 
                     self.bus.fire_event(EVENT_STATE_CHANGED,
-                                        {'category': category,
+                                        {'entity_id': entity_id,
                                          'old_state': old_state,
-                                         'new_state': self.states[category]})
+                                         'new_state': self.states[entity_id]})
 
-    def get_state(self, category):
+    def get_state(self, entity_id):
         """ Returns a dict (state, last_changed, attributes) describing
-            the state of the specified category. """
+            the state of the specified entity. """
         with self.lock:
             try:
                 # Make a copy so people won't mutate the state
-                return self.states[category].copy()
+                return self.states[entity_id].copy()
 
             except KeyError:
-                # If category does not exist
+                # If entity does not exist
                 return None
 
-    def is_state(self, category, state):
-        """ Returns True if category exists and is specified state. """
+    def is_state(self, entity_id, state):
+        """ Returns True if entity exists and is specified state. """
         try:
-            return self.get_state(category).state == state
+            return self.get_state(entity_id).state == state
         except AttributeError:
             # get_state returned None
             return False
@@ -438,8 +438,10 @@ class Timer(threading.Thread):
 
         last_fired_on_second = -1
 
+        calc_now = dt.datetime.now
+
         while True:
-            now = dt.datetime.now()
+            now = calc_now()
 
             # First check checks if we are not on a second matching the
             # timer interval. Second check checks if we did not already fire
@@ -457,7 +459,7 @@ class Timer(threading.Thread):
 
                 time.sleep(slp_seconds)
 
-                now = dt.datetime.now()
+                now = calc_now()
 
             last_fired_on_second = now.second
 
diff --git a/homeassistant/bootstrap.py b/homeassistant/bootstrap.py
index 10396475cbd..a96da96e207 100644
--- a/homeassistant/bootstrap.py
+++ b/homeassistant/bootstrap.py
@@ -138,10 +138,10 @@ def from_config_file(config_path):
 
     # Init groups
     if has_section("groups"):
-        for name, categories in config.items("groups"):
+        for name, entity_ids in config.items("groups"):
             add_status("Group - {}".format(name),
                        group.setup(bus, statemachine, name,
-                                   categories.split(",")))
+                                   entity_ids.split(",")))
 
     # Light trigger
     if light_control:
diff --git a/homeassistant/components/__init__.py b/homeassistant/components/__init__.py
index 0336fba5993..168823c7b0f 100644
--- a/homeassistant/components/__init__.py
+++ b/homeassistant/components/__init__.py
@@ -8,7 +8,7 @@ Component design guidelines:
 
 Each component defines a constant DOMAIN that is equal to its filename.
 
-Each component that tracks states should create state category names in the
+Each component that tracks states should create state entity names in the
 format "<DOMAIN>.<OBJECT_ID>".
 
 Each component should publish services only under its own domain.
diff --git a/homeassistant/components/chromecast.py b/homeassistant/components/chromecast.py
index a2384c09153..a50ae3a994d 100644
--- a/homeassistant/components/chromecast.py
+++ b/homeassistant/components/chromecast.py
@@ -16,7 +16,7 @@ DOMAIN = "chromecast"
 
 SERVICE_YOUTUBE_VIDEO = "play_youtube_video"
 
-STATE_CATEGORY_FORMAT = DOMAIN + '.{}'
+ENTITY_ID_FORMAT = DOMAIN + '.{}'
 STATE_NO_APP = "none"
 
 ATTR_FRIENDLY_NAME = "friendly_name"
@@ -29,15 +29,15 @@ def turn_off(statemachine, cc_id=None):
     """ Exits any running app on the specified ChromeCast and shows
     idle screen. Will quit all ChromeCasts if nothing specified. """
 
-    cats = [STATE_CATEGORY_FORMAT.format(cc_id)] if cc_id \
-        else ha.filter_categories(statemachine.categories, DOMAIN)
+    entity_ids = [ENTITY_ID_FORMAT.format(cc_id)] if cc_id \
+        else ha.filter_entity_ids(statemachine.entity_ids, DOMAIN)
 
-    for cat in cats:
-        state = statemachine.get_state(cat)
+    for entity_id in entity_ids:
+        state = statemachine.get_state(entity_id)
 
-        if state and \
-           state.state != STATE_NO_APP or \
-           state.state != pychromecast.APP_ID_HOME:
+        if (state and
+            (state.state != STATE_NO_APP or
+             state.state != pychromecast.APP_ID_HOME)):
 
             pychromecast.quit_app(state.attributes[ATTR_HOST])
 
@@ -53,7 +53,7 @@ def setup(bus, statemachine, host):
         logger.error("Could not find Chromecast")
         return False
 
-    category = STATE_CATEGORY_FORMAT.format(util.slugify(
+    entity = ENTITY_ID_FORMAT.format(util.slugify(
         device.friendly_name))
 
     bus.register_service(DOMAIN, ha.SERVICE_TURN_OFF,
@@ -80,7 +80,7 @@ def setup(bus, statemachine, host):
         status = pychromecast.get_app_status(host)
 
         if status:
-            statemachine.set_state(category, status.name,
+            statemachine.set_state(entity, status.name,
                                    {ATTR_FRIENDLY_NAME:
                                        pychromecast.get_friendly_name(
                                            status.name),
@@ -88,7 +88,7 @@ def setup(bus, statemachine, host):
                                     ATTR_STATE: status.state,
                                     ATTR_OPTIONS: status.options})
         else:
-            statemachine.set_state(category, STATE_NO_APP, {ATTR_HOST: host})
+            statemachine.set_state(entity, STATE_NO_APP, {ATTR_HOST: host})
 
     ha.track_time_change(bus, update_chromecast_state)
 
diff --git a/homeassistant/components/device_sun_light_trigger.py b/homeassistant/components/device_sun_light_trigger.py
index d3867cec9a8..c2a69c5ae7e 100644
--- a/homeassistant/components/device_sun_light_trigger.py
+++ b/homeassistant/components/device_sun_light_trigger.py
@@ -22,20 +22,20 @@ def setup(bus, statemachine, light_group=None):
 
     logger = logging.getLogger(__name__)
 
-    device_state_categories = ha.filter_categories(statemachine.categories,
-                                                   device_tracker.DOMAIN)
+    device_entity_ids = ha.filter_entity_ids(statemachine.entity_ids,
+                                             device_tracker.DOMAIN)
 
-    if not device_state_categories:
+    if not device_entity_ids:
         logger.error("LightTrigger:No devices found to track")
 
         return False
 
     if not light_group:
-        light_group = light.STATE_GROUP_NAME_ALL_LIGHTS
+        light_group = light.GROUP_NAME_ALL_LIGHTS
 
     # Get the light IDs from the specified group
-    light_ids = ha.filter_categories(
-        group.get_categories(statemachine, light_group), light.DOMAIN, True)
+    light_ids = ha.filter_entity_ids(
+        group.get_entity_ids(statemachine, light_group), light.DOMAIN, True)
 
     if not light_ids:
         logger.error("LightTrigger:No lights found to turn on ")
@@ -48,7 +48,7 @@ def setup(bus, statemachine, light_group=None):
          len(light_ids))
 
     # pylint: disable=unused-argument
-    def handle_sun_rising(category, old_state, new_state):
+    def handle_sun_rising(entity, old_state, new_state):
         """The moment sun sets we want to have all the lights on.
            We will schedule to have each light start after one another
            and slowly transition in."""
@@ -76,7 +76,7 @@ def setup(bus, statemachine, light_group=None):
 
     # Track every time sun rises so we can schedule a time-based
     # pre-sun set event
-    ha.track_state_change(bus, sun.STATE_CATEGORY, handle_sun_rising,
+    ha.track_state_change(bus, sun.ENTITY_ID, handle_sun_rising,
                           sun.STATE_BELOW_HORIZON, sun.STATE_ABOVE_HORIZON)
 
     # If the sun is already above horizon
@@ -84,14 +84,14 @@ def setup(bus, statemachine, light_group=None):
     if sun.is_up(statemachine):
         handle_sun_rising(None, None, None)
 
-    def handle_device_state_change(category, old_state, new_state):
+    def handle_device_state_change(entity, old_state, new_state):
         """ Function to handle tracked device state changes. """
         lights_are_on = group.is_on(statemachine, light_group)
 
         light_needed = not (lights_are_on or sun.is_up(statemachine))
 
         # Specific device came home ?
-        if (category != device_tracker.STATE_CATEGORY_ALL_DEVICES and
+        if (entity != device_tracker.ENTITY_ID_ALL_DEVICES and
            new_state.state == ha.STATE_HOME):
 
             # These variables are needed for the elif check
@@ -103,7 +103,7 @@ def setup(bus, statemachine, light_group=None):
 
                 logger.info(
                     "Home coming event for {}. Turning lights on".
-                    format(category))
+                    format(entity))
 
                 for light_id in light_ids:
                     light.turn_on(bus, light_id)
@@ -127,7 +127,7 @@ def setup(bus, statemachine, light_group=None):
                         break
 
         # Did all devices leave the house?
-        elif (category == device_tracker.STATE_CATEGORY_ALL_DEVICES and
+        elif (entity == device_tracker.ENTITY_ID_ALL_DEVICES and
               new_state.state == ha.STATE_NOT_HOME and lights_are_on):
 
             logger.info(
@@ -136,12 +136,12 @@ def setup(bus, statemachine, light_group=None):
             general.shutdown_devices(bus, statemachine)
 
     # Track home coming of each seperate device
-    for category in device_state_categories:
-        ha.track_state_change(bus, category, handle_device_state_change,
+    for entity in device_entity_ids:
+        ha.track_state_change(bus, entity, handle_device_state_change,
                               ha.STATE_NOT_HOME, ha.STATE_HOME)
 
     # Track when all devices are gone to shut down lights
-    ha.track_state_change(bus, device_tracker.STATE_CATEGORY_ALL_DEVICES,
+    ha.track_state_change(bus, device_tracker.ENTITY_ID_ALL_DEVICES,
                           handle_device_state_change, ha.STATE_HOME,
                           ha.STATE_NOT_HOME)
 
diff --git a/homeassistant/components/device_tracker.py b/homeassistant/components/device_tracker.py
index 1a56415d829..e34bf090913 100644
--- a/homeassistant/components/device_tracker.py
+++ b/homeassistant/components/device_tracker.py
@@ -24,11 +24,11 @@ DOMAIN = "device_tracker"
 
 SERVICE_DEVICE_TRACKER_RELOAD = "reload_devices_csv"
 
-STATE_GROUP_NAME_ALL_DEVICES = 'all_tracked_devices'
-STATE_CATEGORY_ALL_DEVICES = group.STATE_CATEGORY_FORMAT.format(
-    STATE_GROUP_NAME_ALL_DEVICES)
+GROUP_NAME_ALL_DEVICES = 'all_tracked_devices'
+ENTITY_ID_ALL_DEVICES = group.ENTITY_ID_FORMAT.format(
+    GROUP_NAME_ALL_DEVICES)
 
-STATE_CATEGORY_FORMAT = DOMAIN + '.{}'
+ENTITY_ID_FORMAT = DOMAIN + '.{}'
 
 # After how much time do we consider a device not home if
 # it does not show up on scans
@@ -43,10 +43,10 @@ KNOWN_DEVICES_FILE = "known_devices.csv"
 
 def is_home(statemachine, device_id=None):
     """ Returns if any or specified device is home. """
-    category = STATE_CATEGORY_FORMAT.format(device_id) if device_id \
-        else STATE_CATEGORY_ALL_DEVICES
+    entity = ENTITY_ID_FORMAT.format(device_id) if device_id \
+        else ENTITY_ID_ALL_DEVICES
 
-    return statemachine.is_state(category, ha.STATE_HOME)
+    return statemachine.is_state(entity, ha.STATE_HOME)
 
 
 # pylint: disable=too-many-instance-attributes
@@ -83,14 +83,14 @@ class DeviceTracker(object):
 
         self.update_devices(device_scanner.scan_devices())
 
-        group.setup(bus, statemachine, STATE_GROUP_NAME_ALL_DEVICES,
-                    list(self.device_state_categories))
+        group.setup(bus, statemachine, GROUP_NAME_ALL_DEVICES,
+                    list(self.device_entity_ids))
 
     @property
-    def device_state_categories(self):
-        """ Returns a set containing all categories
-            that are maintained for devices. """
-        return set([self.known_devices[device]['category'] for device
+    def device_entity_ids(self):
+        """ Returns a set containing all device entity ids
+            that are being tracked. """
+        return set([self.known_devices[device]['entity_id'] for device
                     in self.known_devices
                     if self.known_devices[device]['track']])
 
@@ -111,7 +111,7 @@ class DeviceTracker(object):
                 self.known_devices[device]['last_seen'] = now
 
                 self.statemachine.set_state(
-                    self.known_devices[device]['category'], ha.STATE_HOME)
+                    self.known_devices[device]['entity_id'], ha.STATE_HOME)
 
         # For all devices we did not find, set state to NH
         # But only if they have been gone for longer then the error time span
@@ -122,7 +122,7 @@ class DeviceTracker(object):
                self.error_scanning):
 
                 self.statemachine.set_state(
-                    self.known_devices[device]['category'],
+                    self.known_devices[device]['entity_id'],
                     ha.STATE_NOT_HOME)
 
         # If we come along any unknown devices we will write them to the
@@ -180,9 +180,9 @@ class DeviceTracker(object):
             with open(KNOWN_DEVICES_FILE) as inp:
                 default_last_seen = datetime(1990, 1, 1)
 
-                # Temp variable to keep track of which categories we use
-                # so we can ensure we have unique categories.
-                used_categories = []
+                # Temp variable to keep track of which entity ids we use
+                # so we can ensure we have unique entity ids.
+                used_entity_ids = []
 
                 try:
                     for row in csv.DictReader(inp):
@@ -195,23 +195,23 @@ class DeviceTracker(object):
                             row['last_seen'] = default_last_seen
 
                             # Make sure that each device is mapped
-                            # to a unique category name
+                            # to a unique entity_id name
                             name = util.slugify(row['name']) if row['name'] \
                                 else "unnamed_device"
 
-                            category = STATE_CATEGORY_FORMAT.format(name)
+                            entity_id = ENTITY_ID_FORMAT.format(name)
                             tries = 1
 
-                            while category in used_categories:
+                            while entity_id in used_entity_ids:
                                 tries += 1
 
                                 suffix = "_{}".format(tries)
 
-                                category = STATE_CATEGORY_FORMAT.format(
+                                entity_id = ENTITY_ID_FORMAT.format(
                                     name + suffix)
 
-                            row['category'] = category
-                            used_categories.append(category)
+                            row['entity_id'] = entity_id
+                            used_entity_ids.append(entity_id)
 
                         known_devices[device] = row
 
@@ -220,21 +220,21 @@ class DeviceTracker(object):
                             "No devices to track. Please update {}.".format(
                                 KNOWN_DEVICES_FILE))
 
-                    # Remove categories that are no longer maintained
-                    new_categories = set([known_devices[device]['category']
+                    # Remove entities that are no longer maintained
+                    new_entity_ids = set([known_devices[device]['entity_id']
                                          for device in known_devices
                                          if known_devices[device]['track']])
 
-                    for category in \
-                            self.device_state_categories - new_categories:
+                    for entity_id in \
+                            self.device_entity_ids - new_entity_ids:
 
                         self.logger.info(
-                            "DeviceTracker:Removing category {}".format(
-                                category))
-                        self.statemachine.remove_category(category)
+                            "DeviceTracker:Removing entity {}".format(
+                                entity_id))
+                        self.statemachine.remove_entity(entity_id)
 
                     # File parsed, warnings given if necessary
-                    # categories cleaned up, make it available
+                    # entities cleaned up, make it available
                     self.known_devices = known_devices
 
                     self.logger.info(
diff --git a/homeassistant/components/group.py b/homeassistant/components/group.py
index 628bb6fb663..7817d39c235 100644
--- a/homeassistant/components/group.py
+++ b/homeassistant/components/group.py
@@ -11,9 +11,9 @@ import homeassistant as ha
 
 DOMAIN = "group"
 
-STATE_CATEGORY_FORMAT = DOMAIN + ".{}"
+ENTITY_ID_FORMAT = DOMAIN + ".{}"
 
-STATE_ATTR_CATEGORIES = "categories"
+STATE_ATTR_ENTITY_IDS = "entity_ids"
 
 _GROUP_TYPES = {
     "on_off": (ha.STATE_ON, ha.STATE_OFF),
@@ -46,29 +46,32 @@ def is_on(statemachine, group):
         return False
 
 
-def get_categories(statemachine, group):
-    """ Get the categories that make up this group. """
-    state = statemachine.get_state(group)
-
-    return state.attributes[STATE_ATTR_CATEGORIES] if state else []
+def get_entity_ids(statemachine, group):
+    """ Get the entity ids that make up this group. """
+    try:
+        return statemachine.get_state(group).attributes[STATE_ATTR_ENTITY_IDS]
+    except (AttributeError, KeyError):
+        # AttributeError if state did not exist
+        # KeyError if key did not exist in attributes
+        return []
 
 
 # pylint: disable=too-many-branches
-def setup(bus, statemachine, name, categories):
+def setup(bus, statemachine, name, entity_ids):
     """ Sets up a group state that is the combined state of
         several states. Supports ON/OFF and DEVICE_HOME/DEVICE_NOT_HOME. """
 
     logger = logging.getLogger(__name__)
 
-    # Loop over the given categories to:
+    # Loop over the given entities to:
     #  - determine which group type this is (on_off, device_home)
     #  - if all states exist and have valid states
     #  - retrieve the current state of the group
     errors = []
     group_type, group_on, group_off, group_state = None, None, None, None
 
-    for cat in categories:
-        state = statemachine.get_state(cat)
+    for entity_id in entity_ids:
+        state = statemachine.get_state(entity_id)
 
         # Try to determine group type if we didn't yet
         if not group_type and state:
@@ -85,15 +88,15 @@ def setup(bus, statemachine, name, categories):
 
                 break
 
-        # Check if category exists
+        # Check if entity exists
         if not state:
-            errors.append("Category {} does not exist".format(cat))
+            errors.append("Entity {} does not exist".format(entity_id))
 
-        # Check if category is valid state
+        # Check if entity is valid state
         elif state.state != group_off and state.state != group_on:
 
             errors.append("State of {} is {} (expected: {}, {})".format(
-                cat, state.state, group_off, group_on))
+                entity_id, state.state, group_off, group_on))
 
         # Keep track of the group state to init later on
         elif group_state == group_off and state.state == group_on:
@@ -105,15 +108,15 @@ def setup(bus, statemachine, name, categories):
 
         return False
 
-    group_cat = STATE_CATEGORY_FORMAT.format(name)
-    state_attr = {STATE_ATTR_CATEGORIES: categories}
+    group_entity_id = ENTITY_ID_FORMAT.format(name)
+    state_attr = {STATE_ATTR_ENTITY_IDS: entity_ids}
 
     # pylint: disable=unused-argument
-    def _update_group_state(category, old_state, new_state):
+    def _update_group_state(entity_id, old_state, new_state):
         """ Updates the group state based on a state change by a tracked
-            category. """
+            entity. """
 
-        cur_group_state = statemachine.get_state(group_cat).state
+        cur_group_state = statemachine.get_state(group_entity_id).state
 
         # if cur_group_state = OFF and new_state = ON: set ON
         # if cur_group_state = ON and new_state = OFF: research
@@ -121,18 +124,18 @@ def setup(bus, statemachine, name, categories):
 
         if cur_group_state == group_off and new_state.state == group_on:
 
-            statemachine.set_state(group_cat, group_on, state_attr)
+            statemachine.set_state(group_entity_id, group_on, state_attr)
 
         elif cur_group_state == group_on and new_state.state == group_off:
 
             # Check if any of the other states is still on
-            if not any([statemachine.is_state(cat, group_on)
-                        for cat in categories if cat != category]):
-                statemachine.set_state(group_cat, group_off, state_attr)
+            if not any([statemachine.is_state(ent_id, group_on)
+                        for ent_id in entity_ids if entity_id != ent_id]):
+                statemachine.set_state(group_entity_id, group_off, state_attr)
 
-    for cat in categories:
-        ha.track_state_change(bus, cat, _update_group_state)
+    for entity_id in entity_ids:
+        ha.track_state_change(bus, entity_id, _update_group_state)
 
-    statemachine.set_state(group_cat, group_state, state_attr)
+    statemachine.set_state(group_entity_id, group_state, state_attr)
 
     return True
diff --git a/homeassistant/components/httpinterface/__init__.py b/homeassistant/components/httpinterface/__init__.py
index 78361f9f37d..eac31972a8c 100644
--- a/homeassistant/components/httpinterface/__init__.py
+++ b/homeassistant/components/httpinterface/__init__.py
@@ -18,31 +18,31 @@ Other status codes that can occur are:
 The api supports the following actions:
 
 /api/states - GET
-Returns a list of categories for which a state is available
+Returns a list of entities for which a state is available
 Example result:
 {
-    "categories": [
+    "entity_ids": [
         "Paulus_Nexus_4",
         "weather.sun",
         "all_devices"
     ]
 }
 
-/api/states/<category> - GET
-Returns the current state from a category
+/api/states/<entity_id> - GET
+Returns the current state from an entity
 Example result:
 {
     "attributes": {
         "next_rising": "07:04:15 29-10-2013",
         "next_setting": "18:00:31 29-10-2013"
     },
-    "category": "weather.sun",
+    "entity_id": "weather.sun",
     "last_changed": "23:24:33 28-10-2013",
     "state": "below_horizon"
 }
 
-/api/states/<category> - POST
-Updates the current state of a category. Returns status code 201 if successful
+/api/states/<entity_id> - POST
+Updates the current state of an entity. Returns status code 201 if successful
 with location header of updated resource and as body the new state.
 parameter: new_state - string
 optional parameter: attributes - JSON encoded object
@@ -52,7 +52,7 @@ Example result:
         "next_rising": "07:04:15 29-10-2013",
         "next_setting": "18:00:31 29-10-2013"
     },
-    "category": "weather.sun",
+    "entity_id": "weather.sun",
     "last_changed": "23:24:33 28-10-2013",
     "state": "below_horizon"
 }
@@ -94,7 +94,7 @@ URL_CHANGE_STATE = "/change_state"
 URL_FIRE_EVENT = "/fire_event"
 
 URL_API_STATES = "/api/states"
-URL_API_STATES_CATEGORY = "/api/states/{}"
+URL_API_STATES_ENTITY = "/api/states/{}"
 URL_API_EVENTS = "/api/events"
 URL_API_EVENTS_EVENT = "/api/events/{}"
 URL_API_SERVICES = "/api/services"
@@ -150,10 +150,10 @@ class RequestHandler(BaseHTTPRequestHandler):
         # /states
         ('GET', '/api/states', '_handle_get_api_states'),
         ('GET',
-         re.compile(r'/api/states/(?P<category>[a-zA-Z\._0-9]+)'),
-         '_handle_get_api_states_category'),
+         re.compile(r'/api/states/(?P<entity_id>[a-zA-Z\._0-9]+)'),
+         '_handle_get_api_states_entity'),
         ('POST',
-         re.compile(r'/api/states/(?P<category>[a-zA-Z\._0-9]+)'),
+         re.compile(r'/api/states/(?P<entity_id>[a-zA-Z\._0-9]+)'),
          '_handle_change_state'),
 
         # /events
@@ -317,8 +317,6 @@ class RequestHandler(BaseHTTPRequestHandler):
             self.server.flash_message = None
 
         # Describe state machine:
-        categories = []
-
         write(("<div class='row'>"
                "<div class='col-xs-12'>"
                "<div class='panel panel-primary'>"
@@ -328,17 +326,15 @@ class RequestHandler(BaseHTTPRequestHandler):
                     "class='form-change-state'>"
                "<input type='hidden' name='api_password' value='{}'>"
                "<table class='table'><tr>"
-               "<th>Category</th><th>State</th>"
+               "<th>Entity ID</th><th>State</th>"
                "<th>Attributes</th><th>Last Changed</th>"
                "</tr>").format(self.server.api_password))
 
-        for category in \
-            sorted(self.server.statemachine.categories,
+        for entity_id in \
+            sorted(self.server.statemachine.entity_ids,
                    key=lambda key: key.lower()):
 
-            categories.append(category)
-
-            state = self.server.statemachine.get_state(category)
+            state = self.server.statemachine.get_state(entity_id)
 
             attributes = "<br>".join(
                 ["{}: {}".format(attr, state.attributes[attr])
@@ -347,14 +343,14 @@ class RequestHandler(BaseHTTPRequestHandler):
             write(("<tr>"
                    "<td>{}</td><td>{}</td><td>{}</td><td>{}</td>"
                    "</tr>").format(
-                  category,
+                  entity_id,
                   state.state,
                   attributes,
                   util.datetime_to_str(state.last_changed)))
 
         # Change state form
-        write(("<tr><td><input name='category' class='form-control' "
-                 "placeholder='Category'></td>"
+        write(("<tr><td><input name='entity_id' class='form-control' "
+                 "placeholder='Entity ID'></td>"
                "<td><input name='new_state' class='form-control' "
                  "placeholder='New State'></td>"
                "<td><textarea rows='3' name='attributes' class='form-control' "
@@ -488,18 +484,18 @@ class RequestHandler(BaseHTTPRequestHandler):
 
     # pylint: disable=invalid-name
     def _handle_change_state(self, path_match, data):
-        """ Handles updating the state of a category.
+        """ Handles updating the state of an entity.
 
         This handles the following paths:
         /change_state
-        /api/states/<category>
+        /api/states/<entity_id>
         """
         try:
             try:
-                category = path_match.group('category')
+                entity_id = path_match.group('entity_id')
             except IndexError:
-                # If group 'category' does not exist in path_match
-                category = data['category'][0]
+                # If group 'entity_id' does not exist in path_match
+                entity_id = data['entity_id'][0]
 
             new_state = data['new_state'][0]
 
@@ -510,21 +506,21 @@ class RequestHandler(BaseHTTPRequestHandler):
                 attributes = None
 
             # Write state
-            self.server.statemachine.set_state(category,
+            self.server.statemachine.set_state(entity_id,
                                                new_state,
                                                attributes)
 
             # Return state if json, else redirect to main page
             if self.use_json:
-                state = self.server.statemachine.get_state(category)
+                state = self.server.statemachine.get_state(entity_id)
 
-                self._write_json(state.to_json_dict(category),
+                self._write_json(state.to_json_dict(entity_id),
                                  status_code=HTTP_CREATED,
                                  location=
-                                 URL_API_STATES_CATEGORY.format(category))
+                                 URL_API_STATES_ENTITY.format(entity_id))
             else:
                 self._message(
-                    "State of {} changed to {}".format(category, new_state))
+                    "State of {} changed to {}".format(entity_id, new_state))
 
         except KeyError:
             # If new_state don't exist in post data
@@ -607,20 +603,20 @@ class RequestHandler(BaseHTTPRequestHandler):
 
     # pylint: disable=unused-argument
     def _handle_get_api_states(self, path_match, data):
-        """ Returns the categories which state is being tracked. """
-        self._write_json({'categories': self.server.statemachine.categories})
+        """ Returns the entitie ids which state are being tracked. """
+        self._write_json({'entity_ids': self.server.statemachine.entity_ids})
 
     # pylint: disable=unused-argument
-    def _handle_get_api_states_category(self, path_match, data):
-        """ Returns the state of a specific category. """
-        category = path_match.group('category')
+    def _handle_get_api_states_entity(self, path_match, data):
+        """ Returns the state of a specific entity. """
+        entity_id = path_match.group('entity_id')
 
-        state = self.server.statemachine.get_state(category)
+        state = self.server.statemachine.get_state(entity_id)
 
         if state:
-            self._write_json(state.to_json_dict(category))
+            self._write_json(state.to_json_dict(entity_id))
         else:
-            # If category does not exist
+            # If entity_id does not exist
             self._message("State does not exist.", HTTP_UNPROCESSABLE_ENTITY)
 
     def _handle_get_api_events(self, path_match, data):
diff --git a/homeassistant/components/light.py b/homeassistant/components/light.py
index 9e3ded597d9..75da3a5eb72 100644
--- a/homeassistant/components/light.py
+++ b/homeassistant/components/light.py
@@ -14,21 +14,21 @@ import homeassistant.components.group as group
 
 DOMAIN = "light"
 
-STATE_GROUP_NAME_ALL_LIGHTS = 'all_lights'
-STATE_CATEGORY_ALL_LIGHTS = group.STATE_CATEGORY_FORMAT.format(
-    STATE_GROUP_NAME_ALL_LIGHTS)
+GROUP_NAME_ALL_LIGHTS = 'all_lights'
+ENTITY_ID_ALL_LIGHTS = group.ENTITY_ID_FORMAT.format(
+    GROUP_NAME_ALL_LIGHTS)
 
-STATE_CATEGORY_FORMAT = DOMAIN + ".{}"
+ENTITY_ID_FORMAT = DOMAIN + ".{}"
 
 MIN_TIME_BETWEEN_SCANS = timedelta(seconds=10)
 
 
 def is_on(statemachine, light_id=None):
     """ Returns if the lights are on based on the statemachine. """
-    category = STATE_CATEGORY_FORMAT.format(light_id) if light_id \
-        else STATE_CATEGORY_ALL_LIGHTS
+    entity_id = ENTITY_ID_FORMAT.format(light_id) if light_id \
+        else ENTITY_ID_ALL_LIGHTS
 
-    return statemachine.is_state(category, ha.STATE_ON)
+    return statemachine.is_state(entity_id, ha.STATE_ON)
 
 
 def turn_on(bus, light_id=None, transition_seconds=None):
@@ -80,21 +80,21 @@ def setup(bus, statemachine, light_control):
                       for light_id in light_control.light_ids}
 
             for light_id, state in status.items():
-                state_category = STATE_CATEGORY_FORMAT.format(light_id)
+                entity_id = ENTITY_ID_FORMAT.format(light_id)
 
                 new_state = ha.STATE_ON if state else ha.STATE_OFF
 
-                statemachine.set_state(state_category, new_state)
+                statemachine.set_state(entity_id, new_state)
 
     ha.track_time_change(bus, update_light_state, second=[0, 30])
 
     update_light_state(None)
 
     # Track the all lights state
-    light_cats = [STATE_CATEGORY_FORMAT.format(light_id) for light_id
+    entity_ids = [ENTITY_ID_FORMAT.format(light_id) for light_id
                   in light_control.light_ids]
 
-    group.setup(bus, statemachine, STATE_GROUP_NAME_ALL_LIGHTS, light_cats)
+    group.setup(bus, statemachine, GROUP_NAME_ALL_LIGHTS, entity_ids)
 
     def handle_light_service(service):
         """ Hande a turn light on or off service call. """
diff --git a/homeassistant/components/sun.py b/homeassistant/components/sun.py
index d52e47c5c02..aae76e7be3d 100644
--- a/homeassistant/components/sun.py
+++ b/homeassistant/components/sun.py
@@ -10,7 +10,7 @@ from datetime import timedelta
 import homeassistant as ha
 import homeassistant.util as util
 
-STATE_CATEGORY = "weather.sun"
+ENTITY_ID = "weather.sun"
 
 STATE_ABOVE_HORIZON = "above_horizon"
 STATE_BELOW_HORIZON = "below_horizon"
@@ -21,12 +21,12 @@ STATE_ATTR_NEXT_SETTING = "next_setting"
 
 def is_up(statemachine):
     """ Returns if the sun is currently up based on the statemachine. """
-    return statemachine.is_state(STATE_CATEGORY, STATE_ABOVE_HORIZON)
+    return statemachine.is_state(ENTITY_ID, STATE_ABOVE_HORIZON)
 
 
 def next_setting(statemachine):
     """ Returns the datetime object representing the next sun setting. """
-    state = statemachine.get_state(STATE_CATEGORY)
+    state = statemachine.get_state(ENTITY_ID)
 
     return None if not state else util.str_to_datetime(
         state.attributes[STATE_ATTR_NEXT_SETTING])
@@ -34,7 +34,7 @@ def next_setting(statemachine):
 
 def next_rising(statemachine):
     """ Returns the datetime object representing the next sun setting. """
-    state = statemachine.get_state(STATE_CATEGORY)
+    state = statemachine.get_state(ENTITY_ID)
 
     return None if not state else util.str_to_datetime(
         state.attributes[STATE_ATTR_NEXT_RISING])
@@ -79,7 +79,7 @@ def setup(bus, statemachine, latitude, longitude):
             STATE_ATTR_NEXT_SETTING: util.datetime_to_str(next_setting_dt)
         }
 
-        statemachine.set_state(STATE_CATEGORY, new_state, state_attributes)
+        statemachine.set_state(ENTITY_ID, new_state, state_attributes)
 
         # +10 seconds to be sure that the change has occured
         ha.track_time_change(bus, update_sun_state,
diff --git a/homeassistant/remote.py b/homeassistant/remote.py
index 87e3fa105b0..a0640ba73ef 100644
--- a/homeassistant/remote.py
+++ b/homeassistant/remote.py
@@ -202,13 +202,13 @@ class StateMachine(ha.StateMachine):
         self.logger = logging.getLogger(__name__)
 
     @property
-    def categories(self):
-        """ List of categories which states are being tracked. """
+    def entity_ids(self):
+        """ List of entity ids which states are being tracked. """
 
         try:
             req = self._call_api(METHOD_GET, hah.URL_API_STATES)
 
-            return req.json()['categories']
+            return req.json()['entity_ids']
 
         except requests.exceptions.ConnectionError:
             self.logger.exception("StateMachine:Error connecting to server")
@@ -218,19 +218,19 @@ class StateMachine(ha.StateMachine):
             self.logger.exception("StateMachine:Got unexpected result")
             return []
 
-        except KeyError:  # If 'categories' key not in parsed json
+        except KeyError:  # If 'entity_ids' key not in parsed json
             self.logger.exception("StateMachine:Got unexpected result (2)")
             return []
 
-    def remove_category(self, category):
+    def remove_entity(self, entity_id):
         """ This method is not implemented for remote statemachine.
 
         Throws NotImplementedError. """
 
         raise NotImplementedError
 
-    def set_state(self, category, new_state, attributes=None):
-        """ Set the state of a category, add category if it does not exist.
+    def set_state(self, entity_id, new_state, attributes=None):
+        """ Set the state of a entity, add entity if it does not exist.
 
         Attributes is an optional dict to specify attributes of this state. """
 
@@ -243,7 +243,7 @@ class StateMachine(ha.StateMachine):
 
         try:
             req = self._call_api(METHOD_POST,
-                                 hah.URL_API_STATES_CATEGORY.format(category),
+                                 hah.URL_API_STATES_ENTITY.format(entity_id),
                                  data)
 
             if req.status_code != 201:
@@ -260,13 +260,12 @@ class StateMachine(ha.StateMachine):
         finally:
             self.lock.release()
 
-    def get_state(self, category):
-        """ Returns a dict (state,last_changed, attributes) describing
-            the state of the specified category. """
+    def get_state(self, entity_id):
+        """ Returns the state of the specified entity. """
 
         try:
             req = self._call_api(METHOD_GET,
-                                 hah.URL_API_STATES_CATEGORY.format(category))
+                                 hah.URL_API_STATES_ENTITY.format(entity_id))
 
             if req.status_code == 200:
                 data = req.json()
@@ -274,7 +273,7 @@ class StateMachine(ha.StateMachine):
                 return ha.State.from_json_dict(data)
 
             elif req.status_code == 422:
-                # Category does not exist
+                # Entity does not exist
                 return None
 
             else:
diff --git a/homeassistant/test.py b/homeassistant/test.py
index 8263c062631..e45991f649d 100644
--- a/homeassistant/test.py
+++ b/homeassistant/test.py
@@ -77,12 +77,12 @@ class TestHTTPInterface(unittest.TestCase):
         """ Test if we get access denied if we omit or provide
             a wrong api password. """
         req = requests.get(
-            _url(hah.URL_API_STATES_CATEGORY.format("test")))
+            _url(hah.URL_API_STATES_ENTITY.format("test")))
 
         self.assertEqual(req.status_code, 401)
 
         req = requests.get(
-            _url(hah.URL_API_STATES_CATEGORY.format("test")),
+            _url(hah.URL_API_STATES_ENTITY.format("test")),
             params={"api_password": "not the password"})
 
         self.assertEqual(req.status_code, 401)
@@ -92,7 +92,7 @@ class TestHTTPInterface(unittest.TestCase):
         self.statemachine.set_state("test.test", "not_to_be_set_state")
 
         requests.post(_url(hah.URL_CHANGE_STATE),
-                      data={"category": "test.test",
+                      data={"entity_id": "test.test",
                             "new_state": "debug_state_change2",
                             "api_password": API_PASSWORD})
 
@@ -122,20 +122,20 @@ class TestHTTPInterface(unittest.TestCase):
 
         self.assertEqual(len(test_value), 1)
 
-    def test_api_list_state_categories(self):
-        """ Test if the debug interface allows us to list state categories. """
+    def test_api_list_state_entities(self):
+        """ Test if the debug interface allows us to list state entities. """
         req = requests.get(_url(hah.URL_API_STATES),
                            data={"api_password": API_PASSWORD})
 
         data = req.json()
 
-        self.assertEqual(self.statemachine.categories,
-                         data['categories'])
+        self.assertEqual(self.statemachine.entity_ids,
+                         data['entity_ids'])
 
     def test_api_get_state(self):
         """ Test if the debug interface allows us to get a state. """
         req = requests.get(
-            _url(hah.URL_API_STATES_CATEGORY.format("test")),
+            _url(hah.URL_API_STATES_ENTITY.format("test")),
             data={"api_password": API_PASSWORD})
 
         data = ha.State.from_json_dict(req.json())
@@ -149,17 +149,17 @@ class TestHTTPInterface(unittest.TestCase):
     def test_api_get_non_existing_state(self):
         """ Test if the debug interface allows us to get a state. """
         req = requests.get(
-            _url(hah.URL_API_STATES_CATEGORY.format("does_not_exist")),
+            _url(hah.URL_API_STATES_ENTITY.format("does_not_exist")),
             params={"api_password": API_PASSWORD})
 
         self.assertEqual(req.status_code, 422)
 
     def test_api_state_change(self):
-        """ Test if we can change the state of a category that exists. """
+        """ Test if we can change the state of an entity that exists. """
 
         self.statemachine.set_state("test.test", "not_to_be_set_state")
 
-        requests.post(_url(hah.URL_API_STATES_CATEGORY.format("test.test")),
+        requests.post(_url(hah.URL_API_STATES_ENTITY.format("test.test")),
                       data={"new_state": "debug_state_change2",
                             "api_password": API_PASSWORD})
 
@@ -167,20 +167,20 @@ class TestHTTPInterface(unittest.TestCase):
                          "debug_state_change2")
 
     # pylint: disable=invalid-name
-    def test_api_state_change_of_non_existing_category(self):
+    def test_api_state_change_of_non_existing_entity(self):
         """ Test if the API allows us to change a state of
-            a non existing category. """
+            a non existing entity. """
 
         new_state = "debug_state_change"
 
         req = requests.post(
-            _url(hah.URL_API_STATES_CATEGORY.format(
-                "test_category_that_does_not_exist")),
+            _url(hah.URL_API_STATES_ENTITY.format(
+                "test_entity_that_does_not_exist")),
             data={"new_state": new_state,
                   "api_password": API_PASSWORD})
 
         cur_state = (self.statemachine.
-                     get_state("test_category_that_does_not_exist").state)
+                     get_state("test_entity_that_does_not_exist").state)
 
         self.assertEqual(req.status_code, 201)
         self.assertEqual(cur_state, new_state)
@@ -326,14 +326,14 @@ class TestRemote(unittest.TestCase):
         cls.sm_with_remote_eb.set_state("test", "a_state")
 
     # pylint: disable=invalid-name
-    def test_remote_sm_list_state_categories(self):
-        """ Test if the debug interface allows us to list state categories. """
+    def test_remote_sm_list_state_entities(self):
+        """ Test if the debug interface allows us to list state entity ids. """
 
-        self.assertEqual(self.statemachine.categories,
-                         self.remote_sm.categories)
+        self.assertEqual(self.statemachine.entity_ids,
+                         self.remote_sm.entity_ids)
 
     def test_remote_sm_get_state(self):
-        """ Test if the debug interface allows us to list state categories. """
+        """ Test if debug interface allows us to get state of an entity. """
         remote_state = self.remote_sm.get_state("test")
 
         state = self.statemachine.get_state("test")
@@ -343,11 +343,11 @@ class TestRemote(unittest.TestCase):
         self.assertEqual(remote_state.attributes, state.attributes)
 
     def test_remote_sm_get_non_existing_state(self):
-        """ Test if the debug interface allows us to list state categories. """
+        """ Test remote state machine to get state of non existing entity. """
         self.assertEqual(self.remote_sm.get_state("test_does_not_exist"), None)
 
     def test_remote_sm_state_change(self):
-        """ Test if we can change the state of a category that exists. """
+        """ Test if we can change the state of an existing entity. """
 
         self.remote_sm.set_state("test", "set_remotely", {"test": 1})