Many mobile apps engage their users by sending push notifications. But scaling up with push notifications when you have millions of users is very challenging. In this talk we will use Windows Azure Notification Hubs, which provide a scalable backend for sending millions of push notifications to your users. Notification Hubs support user segmentation by arbitrary custom tags, so you can send push notifications to fans of a specific sports team or to travelers in a specific city. We will explore this support on all four major platforms – iOS, Android, Windows 8, and Windows Phone.