How can Open Source Software and safety critical systems work together? DFSGmbH relies on Linux for its core air traffic control systems. We havesuccessfully managed to standardize a highly reliable, modular operatingsystem and hardware platform at DFS. In a brief overview we explain the challenges, technical and non-technicalones, we have faced getting to the modular platforms. What approaches did wetake and why? Which ones did work out in the long end? We will cover a broadrange of aspects ranging from hardware life cycles, application lifecycles,regulations, deployment solutions, user acceptance, cost cutting and testing.