Pex is a white-box test generation tool for .NET. Starting from a hand-written parameterized unit test, Pex analyzes the program-under-test to determine relevant test inputs fully automatically. To this end, Pex executes the program multiple times with different inputs while monitoring the taken execution paths. Pex uses a constraint solver to determine new test inputs that will exercise different execution paths. As a side effect, Pex may find errors in the program. The result is a traditional unit test suite with high code coverage. In addition, Pex suggests to the programmer how to fix certain errors. Pex is integrated into Microsoft Visual Studio.