This paper summarises work done at I.N.T. with the Chorus Operating System. It shortly describes Chorus' concepts and abstractions useful to understand our work. Then, it focuses on Chorus micro kernel capabilities and explains how these capabilities have been used to allow cohabitation between the Chorus micro kernel and the Macintosh Operating system. It also describes how a~new subsystem can be built over the micro kernel. Finally, we discuss some limitations of Chorus, especially related to the support of large networks, and how the system must be extended to address this new feature.