diff --git a/build/css/main.css b/build/css/main.css
index d84f8e32..b7e7f080 100644
--- a/build/css/main.css
+++ b/build/css/main.css
@@ -6269,6 +6269,8 @@ body {
 
 .modal-text {
   padding: 20px; }
+  .modal-text > p {
+    white-space: pre-line; }
   .modal-text > p:last-child {
     margin-bottom: 0; }
 
diff --git a/lib/gui/app.js b/lib/gui/app.js
index 1f106920..b7077e2d 100644
--- a/lib/gui/app.js
+++ b/lib/gui/app.js
@@ -23,6 +23,8 @@
 /* eslint-disable no-var */
 
 var angular = require('angular');
+const electron = require('electron');
+const EXIT_CODES = require('../src/exit-codes');
 
 /* eslint-enable no-var */
 
@@ -48,6 +50,7 @@ const app = angular.module('Etcher', [
   require('./components/svg-icon/svg-icon'),
   require('./components/update-notifier/update-notifier'),
   require('./components/drive-selector/drive-selector'),
+  require('./components/warning-modal/warning-modal'),
 
   // Pages
   require('./pages/main/main'),
@@ -145,6 +148,45 @@ app.run(($timeout, DriveScannerService, DrivesModel, ErrorService, DriveSelector
   DriveScannerService.start();
 });
 
+app.run(($window, WarningModalService, ErrorService, FlashStateModel) => {
+  let popupExists = false;
+
+  $window.addEventListener('beforeunload', (event) => {
+
+    // Don't close window while flashing
+    if (FlashStateModel.isFlashing()) {
+      event.returnValue = false;
+    }
+
+    if (popupExists) {
+      return;
+    }
+
+    // Don't open any more popups
+    popupExists = true;
+
+    WarningModalService.display({
+      confirmationLabel: 'Yes, quit',
+      rejectionLabel: 'Cancel',
+      description: [
+        'You are currently flashing a drive. Closing Etcher may leave',
+        'your drive in an unusable state.\n\nAre you sure you want to',
+        'close Etcher?'
+      ].join(' ')
+    }).then((confirmed) => {
+      if (confirmed) {
+
+        // This circumvents the 'beforeunload' event unlike
+        // electron.remote.app.quit() which does not.
+        electron.remote.process.exit(EXIT_CODES.SUCCESS);
+
+      }
+
+      popupExists = false;
+    }).catch(ErrorService.reportException);
+  });
+});
+
 app.config(($urlRouterProvider) => {
   $urlRouterProvider.otherwise('/main');
 });
diff --git a/lib/gui/components/modal/styles/_modal.scss b/lib/gui/components/modal/styles/_modal.scss
index e40f1f96..1b98c45a 100644
--- a/lib/gui/components/modal/styles/_modal.scss
+++ b/lib/gui/components/modal/styles/_modal.scss
@@ -92,6 +92,10 @@
 .modal-text {
   padding: 20px;
 
+  > p {
+    white-space: pre-line;
+  }
+
   > p:last-child {
     margin-bottom: 0;
   }
diff --git a/lib/gui/components/warning-modal/services/warning-modal.js b/lib/gui/components/warning-modal/services/warning-modal.js
index c3ec7676..befdf699 100644
--- a/lib/gui/components/warning-modal/services/warning-modal.js
+++ b/lib/gui/components/warning-modal/services/warning-modal.js
@@ -27,7 +27,8 @@ module.exports = function(ModalService) {
    *
    * @param {Object} options - options
    * @param {String} options.description - danger message
-   * @param {String} options.confirmationLabel - danger message
+   * @param {String} options.confirmationLabel - confirmation button text
+   * @param {String} options.rejectionLabel - rejection button text
    * @fulfil {Boolean} - whether the user accepted or rejected the warning
    * @returns {Promise}
    *
diff --git a/lib/gui/components/warning-modal/templates/warning-modal.tpl.html b/lib/gui/components/warning-modal/templates/warning-modal.tpl.html
index 0a67df77..61b7383c 100644
--- a/lib/gui/components/warning-modal/templates/warning-modal.tpl.html
+++ b/lib/gui/components/warning-modal/templates/warning-modal.tpl.html
@@ -13,7 +13,11 @@
 </div>
 
 <div class="modal-footer">
-  <button class="button button-danger button-block"
-    ng-click="modal.accept()">{{ ::modal.options.confirmationLabel }}</button>
+  <div class="modal-menu">
+    <button class="button button-danger button-block"
+      ng-click="modal.accept()">{{ ::modal.options.confirmationLabel }}</button>
+    <button ng-if="modal.options.rejectionLabel" class="button button-block"
+      ng-click="modal.reject()">{{ ::modal.options.rejectionLabel }}</button>
+  </div>
 </div>