Index: src/wp-admin/css/customize-controls.css
===================================================================
--- src/wp-admin/css/customize-controls.css	(revision 36256)
+++ src/wp-admin/css/customize-controls.css	(working copy)
@@ -1311,6 +1311,117 @@
 	color: #00a0d2;
 }
 
+/* Device/preview size toggles */
+
+.wp-full-overlay {
+	background: #191e23;
+}
+
+#customize-preview.wp-full-overlay-main {
+	background-color: #f1f1f1;
+}
+
+.expanded #customize-footer-actions {
+	position: fixed;
+	bottom: 0;
+	left: 0;
+	width: 300px;
+	height: 45px;
+	border-top: 1px solid #ddd;
+}
+
+#customize-footer-actions .devices {
+	float: right;
+}
+
+#customize-footer-actions .devices button {
+	cursor: pointer;
+	background: transparent;
+	border: none;
+	padding: 0;
+	box-shadow: none;
+	-webkit-transition: color .1s ease-in-out;
+	transition: color .1s ease-in-out;
+}
+
+#customize-footer-actions .devices button:focus {
+	box-shadow: none;
+	outline: none;
+}
+
+#customize-footer-actions .devices button:before {
+	display: inline-block;
+	-webkit-font-smoothing: antialiased;
+	font: normal 20px/30px "dashicons";
+	vertical-align: top;
+	margin: 3px 1px;
+	padding: 4px 8px;
+	border-radius: 100%;
+	color: #656a6f; /* #82878c; would be better but doesn't meet color contrast guidelines */
+}
+
+#customize-footer-actions .devices button.active:before {
+	color: #191e23;
+	box-shadow: 0 0 2px rgba(0, 0, 0, .75);
+}
+
+#customize-footer-actions .devices button:hover:before,
+#customize-footer-actions .devices button:focus:before {
+	color: #191e23;
+	box-shadow: 0 0 0 1px #5b9dd9,
+	            0 0 2px 1px rgba(30, 140, 190, .8);
+}
+
+.wp-core-ui .wp-full-overlay .collapse-sidebar:hover,
+.wp-core-ui .wp-full-overlay .collapse-sidebar:focus {
+	color: #191e23;
+}
+
+#customize-footer-actions .devices .preview-desktop:before {
+	content: "\f472";
+}
+
+#customize-footer-actions .devices .preview-tablet:before {
+	content: "\f471";
+}
+
+#customize-footer-actions .devices .preview-mobile:before {
+	content: "\f470";
+}
+
+@media screen and (max-width:1024px) {
+	#customize-footer-actions .devices {
+		display: none;
+	}
+}
+
+.collapsed #customize-footer-actions .devices button:before {
+	display: none;
+}
+
+.collapsed #customize-footer-actions .devices .preview-full {
+	left: 0;
+}
+
+.preview-mobile #customize-preview {
+	margin: auto 0 auto -160px;
+	width: 320px;
+	height: 480px;
+	max-height: 100%;
+	max-width: 100%;
+	left: 50%;
+}
+
+.preview-tablet #customize-preview {
+	margin: auto 0 auto -3in;
+	width: 6in;
+	height: 9in;
+	max-height: 100%;
+	max-width: 100%;
+	left: 50%;
+}
+
+
 /* Responsive */
 .customize-controls-preview-toggle {
 	display: none;
Index: src/wp-admin/customize.php
===================================================================
--- src/wp-admin/customize.php	(revision 36256)
+++ src/wp-admin/customize.php	(working copy)
@@ -137,7 +137,7 @@
 					<button class="customize-help-toggle dashicons dashicons-editor-help" aria-expanded="false"><span class="screen-reader-text"><?php _e( 'Help' ); ?></span></button>
 				</div>
 				<div class="customize-panel-description"><?php
-					_e( 'The Customizer allows you to preview changes to your site before publishing them. You can also navigate to different pages on your site to preview them.' );
+					_e( 'The Customizer allows you to preview changes to your site before publishing them. You can navigate to different pages on your site to preview them.' );
 				?></div>
 			</div>
 
@@ -148,6 +148,11 @@
 		</div>
 
 		<div id="customize-footer-actions" class="wp-full-overlay-footer">
+			<div class="devices">
+				<button type="button" class="preview-desktop active" aria-pressed="true" data-device="desktop"><span class="screen-reader-text"><?php _e( 'Enter desktop preview mode' ); ?></span></button>
+				<button type="button" class="preview-tablet" aria-pressed="false" data-device="tablet"><span class="screen-reader-text"><?php _e( 'Enter tablet preview mode' ); ?></span></button>
+				<button type="button" class="preview-mobile" aria-pressed="false" data-device="mobile"><span class="screen-reader-text"><?php _e( 'Enter mobile preview mode' ); ?></span></button>
+			</div>
 			<button type="button" class="collapse-sidebar button-secondary" aria-expanded="true" aria-label="<?php esc_attr_e( 'Collapse Sidebar' ); ?>">
 				<span class="collapse-sidebar-arrow"></span>
 				<span class="collapse-sidebar-label"><?php _e( 'Collapse' ); ?></span>
Index: src/wp-admin/js/customize-controls.js
===================================================================
--- src/wp-admin/js/customize-controls.js	(revision 36256)
+++ src/wp-admin/js/customize-controls.js	(working copy)
@@ -3602,6 +3602,19 @@
 			event.preventDefault();
 		});
 
+		// Preview size toggles.
+		$( '#customize-footer-actions .devices button' ).on( 'click', function( event ) {
+			var overlay = $( '.wp-full-overlay' ),
+			    device = $( event.currentTarget ).data( 'device' );
+			$( '#customize-footer-actions .devices button' ).removeClass( 'active' )
+			                                                .attr( 'aria-pressed', false );
+			overlay.removeClass( 'preview-full preview-desktop preview-tablet preview-mobile' )
+			       .toggleClass( 'preview-' + device );
+			$( event.currentTarget ).addClass( 'active' )
+			                        .attr( 'aria-pressed', true );
+			api.previewedDevice = device;
+		});
+
 		// Bind site title display to the corresponding field.
 		if ( title.length ) {
 			$( '#customize-control-blogname input' ).on( 'input', function() {
Index: src/wp-includes/class-wp-customize-manager.php
===================================================================
--- src/wp-includes/class-wp-customize-manager.php	(revision 36256)
+++ src/wp-includes/class-wp-customize-manager.php	(working copy)
@@ -1694,6 +1694,7 @@
 			'autofocus' => array(),
 			'documentTitleTmpl' => $this->get_document_title_template(),
 		);
+		$this->add_setting( 'previewedDevice', array( 'default' => 'desktop' ) );
 
 		// Prepare Customize Section objects to pass to JavaScript.
 		foreach ( $this->sections() as $id => $section ) {