During development and testing it is often needed to test different kernels or run various sets of unit tests quickly. With lrun it is possible to do exactly that. It utilizes existing technology including UML, KVM and Namespaces to facility different environments. It has been in active use for testing Bluetooth and Wi-Fi features on Linux and can be easily extended to other technologies in the future. This presentation will introduce lrun and its design. It will also show demos of its current use cases.