'No JUnit tests found' in Eclipse

If none of the other answers work for you, here's what worked for me.

Restart eclipse

I had source folder configured correctly, and unit tests correctly annotated but was still getting "No JUnit tests found", for one project. After a restart it worked. I was using STS 3.6.2 based of eclipse Luna 4.4.1

Right Click on Project > Properties > Java Build Path > Add the Test folder as source folder.

All source folders including Test Classes need to be in Eclipse Java Build Path. So that the sources such as main and test classes can be compiled into the build directory (Eclipse default folder is bin).