How to show an Activity as pop-up on other Activity?

For AppCompat, add

android:theme="@style/Theme.AppCompat.Dialog.Alert"

to the activity in AndroidManifest

Just to add on oli's answer, make sure to use the Dialog from the theme you are using in your application.

In my case I did android:theme="@android:style/Theme.Holo.Light.Dialog"

If you want to do this using an Activity instead of a Dialog, you can do this by setting the activity's theme to android:theme="@android:style/Theme.Dialog" in the manifest - this will make the activity appear like a dialog (floating on top of whatever was underneath it).

The Dialog class is perfect to do that. you can find easy examples here.