Mach is an operating system microkernel developed at Carnegie Mellon University to support operating system research, primarily distributed and parallel computation. It is one of the earliest examples of a microkernel, and still the standard by which similar projects are measured.

The project at Carnegie Mellon ran from 1985 to 1994, ending with Mach 3.0. A number of other efforts have continued Mach research, including the University of Utah's blank">Mach 4. Mach was developed as a replacement for the kernel in the _BSD version of UNIX, so no new operating system would have to be designed around it. Today further experimental research on Mach appears ended, although Mach and its derivatives are in use in a number of commercial operating systems, such as NEXTSTEP and OPENSTEP, and most notably Mac OS X (using the XNU kernel). The Mach VM system was also adopted by the BSD developers at CSRG, and appears in modern BSD-derived UNIX systems, such as FreeBSD. Neither Mac OS X nor FreeBSD maintain the microkernel structure pioneered in Mach, although Mac OS X continues to offer microkernel Inter-Process Communication and control primitives for use directly by applications.

Mach is the logical successor to Carnegie Mellon's Accent kernel. The lead developer on the Mach project, Richard Rashid, has been working at Microsoft since 1991 in various top-level positions revolving around the Microsoft Research division. Another of the original Mach developers, Avie Tevanian, was formerly head of software at NeXT, then Chief Software Technology Officer at Apple Computer until March 2006.http://sfgate.com/cgi-bin/blogs/sfgate/sso_detail?blogid=19&entry_id=3821