How to force Android Studio to start with the welcome screen?

Go to

(as of July 25, 2015)
File -> Settings -> Appearance & Behavior -> System Settings -> Un-check "Reopen last project on startup" -> Click apply

(old version)
File -> Settings -> General -> Startup/shutdown -> Reopen last project on startup.

Un-Check the checkbox.

Or, on the Mac, Preferences -> General -> ....

It is now not under File, but in the main menu, goto: Android Studio -> Preferences -> Appearance & Behavior -> Click on: System Settings -> Then you see: Un-check "Reopen last project on startup" -> Click apply

For Mac users, as of Version 4.1.2, the path is Preferences (CMD + ,) -> Appearance & Behavior -> System Settings -> uncheck Reopen last project on startup.