Android Studio cannot resolve org.junit

Try File -> Invalidate Caches / Restart.

Put this in your gradle file:

 testCompile 'junit:junit:4.12'

If none of the above work, it might be that your test classes are in the wrong directory.

I ran into this issue because my text classes were in the /main/java dir instead of the /test dir where they're supposed to be...

In short:

Make sure your test classes are in the /test directory!