Why does visual studio 2012 not find my tests?

I had same symptoms, but under different circumstances.

I had to add one additional step to Peter Lamberg's solution — Clean your solution/project.

My unittest project targets x64. When I created the project it was originally targeting x86.

After switching to x64 all my unit tests disappeared.

I had to go to the Test Menu -> Test Setting -Default Processor Architecture -> x64.

They still didn't show up.

Did a build.

Still didn't show up.

Finally did a Clean

Then they showed up.

I find Clean Solution and Clean to be quite useful at getting the solutions to play ball when setting have changed. Sometimes I have to go to the extreme and delete the obj and bin directories and do a rebuild.

Please add the keyword public to your class definition. Your test class is currently not visible outside its own assembly.

namespace tests {
    [TestClass]
    public class SimpleTest {
        [TestMethod]
        public void Test() {
            Assert.AreEqual("a","a", "same");
        }
    }
}

This sometimes works.

Check that the processor architecture under Test menu matches the one you use to build the solution.

Test -> Test Settings -> Default Processor Architecture -> x86 / x64

As mentioned in other posts, make sure you have the Test Explorer window open. Test -> Windows -> Test Explorer

Then rebuilding the project with the tests should make the tests appear in Test Explorer.

Edit: As Ourjamie pointed out below, doing a clean build may also help. In addition to that, here is one more thing I encountered:

The "Build" checkbox was unticked in Configuration Manager for a new test project I had created under the solution.

Go to Build -> Configuration Manager. Make sure your test project has build checkbox checked for all solution configurations and solution platforms.

I have Visual Studio 2012 and i couldn't see the Tests in Test Explorer,

So I installed the following: NUnit Test Adapter

That fixed the issue for me !