In the talk we will present an overview about the L4 microkernel family as well as systems built on top of it. We will focus on L4Linux and secure systems. The distinguished feature of L4 microkernels is their limitiation to the most primitive abstractions. All other functionality is implemented in user land. The system design allows that system core components can be isolated by address spaces. L4Linux is a port of the Linux kernel to the L4 microkernel. L4Linux runs as a user space application side by side with other L4 programs on the microkernel system. We will present the design and the most interesting implementation details.Building secure systems on top of microkernels exemplifies how kernel design influences the overall characteristics of the whole system. In the second part of the talk we will outline how such a secure system can be constructed and will show an example on top of L4.