This is one thing that Android definitely got right from the beginning. This navigation model (for that matter, the fact that there even *is* a model, as compared to the iPhone) all stems from the Activity/Intention architecture baked into Android. There's not so much an app as a collection of activities, and intentions to link them together. This architecture causes a good navigation model to fall out naturally, and makes it easier to construct apps in such a way that they integrate better with other apps, and have a lifecycle that is more resilient to being terminated under low-memory conditions and such. - Joel Webber

It took a while before I internalized that back isn't meant to navigate within an app, it's meant to be a global back button. I still make that mistake pretty often where I want to go to a higher level screen within an app, but press back instead of the internal navigation controls. It doesn't help that the internal navigation is usually inconsistent from app to app, and that the back button _usually_ gets me to the right place (just mentally reinforcing the error.) - Ryan Moulton

For better or worse, apps can also override the default behavior of the back button. - Evan Parker