Android's system UI provides the framework on top of which you build your app, whether you're designing for phones, tablets, watches, or other form factors. Aspects of UI that are especially important for phones and tablets include the Home screen experience, global device navigation, and notifications.
This page introduces these essential UI elements for phones and tablets with links for more information. If you'd like to see the complete designer guide, visit the material design guidelines.
Home, All Apps, and Recents

Home screen
Home is a customizable space that houses app shortcuts, folders and widgets.

All apps screen
The All Apps screen lets users browse the entire set of apps that are installed on your device.

Recents screen
The Recents screen provides an efficient way of switching between recently used apps.
System bars
The system bars are screen areas dedicated to the display of notifications, communication of device status, and device navigation. Typically the system bars are displayed concurrently with your app. Apps that display immersive content, such as movies or images, can temporarily hide the system bars to allow the user to enjoy full screen content without distraction.

Status Bar
Displays app notifications and system status information.

Navigation Bar
Provides the device navigation controls Back, Home, and Recents.
Learn about controlling the system UI
Notifications
Notifications are brief messages that users can access at any time from the status bar. They provide updates, reminders, or information that's important. Touching a notification opens the associated app.
More design resources
For much more about designing an app for Android devices, see the following pages from the material design guidelines.