This post explains Windows Phone application execution model by taking you step-by-step through the different aspects of launching, running, and closing a Silverlight app. Note that for XNA games the terms and ideas remain the same with some minor implementation differences. "With Windows Phone developer tools Beta, apps running on WP employ an updated execution model. Now, when your app is removed from phone foreground (a different app occupies phone foreground like incoming phone call, or lock screen being activated), it's actually being terminated. In earlier version of WP tools, you could get into all kinds of strange scenarios if you went from one app to another. That's because in previous tools, WP OS didn't terminate your app, it "parked" it. However that led to some issues regarding launching a new instance of your app vs. navigating back to instance on back stack. Adding to confusion, the events and API were less than perfect," explains Yochay Kiriaty.