In this thesis we describe how to systematically test, where our target has been Open Source Systems. We have applied a series of common and overlapped test design techniques at defined levels, specifically using seven different functional and structural test approaches. Our conclusion is that open source systems often lack fundamental testing, where on average it only takes 6 test cases to reveal the first failure. The first time to failure is 1 hour on average and MTTF (mean time between failures) is approximately 2 hours with our systematic approach. Our systematic approach is not only testing in itself, but we do also describe the process of discovering a system’s requirements. We have also found that some test design techniques seem to be more effective than others to find failures. We have investigated fifteen different open source systems, attempting to classify these systems in a methodical way. Our process consists in measuring time spent to identify unique part of the system where to apply the test cases. We consider both the system and the test design technique as measures to evaluate the effectiveness and construct test cases.