Modular architectures based on a microkernel are suitable bases for the design and implementation of operating systems. Prototype systems employing microkernel architectures are achieving the levels of functionality and performance expected and required of commercial products. Researchers at Carnegie Mellon University the Open Software Foundation and other sites are investigating implementations of a number of operating systems (e.g. Unix MS-DOS) that use the Mach microkernel. This paper describes the Mach microkernel its use to support implementations of other operating systems and the status of these efforts.