When running load tests in Visual Studio 2010, you might come across something that mentions the need for “Virtual User” licenses.  This is because, with Visual Studio 2010, each simultaneous “simulated user” needs a Virtual User License.  must be licensed.  If you try to run a load test that requires more virtual users than you have licenses, the load test will fail and return a message indicating that you do not have enough virtual user licenses to run the test. 

Fortunately, each copy of Visual Studio 2010 Ultimate comes with an “implied” license for 250 virtual users.  This will allow you to run load tests with up to 250 “simultaneous” virtual users.  However …, this license is only good when you run load tests through Visual Studio using a “Local” test agent.  The minute you connect the tests to a remote test agent, the “implied” licenses go away.  In order to run a load test against a remote test agent, you must purchase a “Visual Studio Load Test Virtual User Pack 2010.”  For each pack that you purchase, you will be licensed for an additional 1000 virtual users and can use them in local runs done in Visual Studio or remote test runs outside of Visual Studio.