Is it possible to find out when Android app returns from background?

Is it possible to find out when Android app returns from background?

So, the idea is: user launching application, and I have to perform task X.
User work some time and then quit (app is in background now).
Then he open app again (from background) and I have to perform task X again.
In first case, I can simply use onCreate() of custom Application class,
but it is not that simple with the second one - there's no onResume()
method or something. Any ideas?