This book describes the design and intemals of the MOSIX distributed operating system. MOSIX, an acronym for Multicomputer Operating System for UNIX, integrates a cluster of loosely integrated computers into a virtual single-machine UNIX environment. The main property of MOSIX is the high degree of integration among the processors, which may include personal workstations and shared memory and non-shared memory multiprocessors, connected by fast communication links. This integration includes network transparency, co-operation between the processors to provide services across machine boundaries, support of dynamic configuration, and system-initiated load balancing by process migration. Another property of MOSIX is the ability to scale up the system configuration to encompass a large number of computers. The development of MOSIX was begun in 1981. The book is intended primarily for readers who are interested in distributed and multiprocessor systems. The reader is assumed to have some knowledge in programming and operating systems, preferably UNIX. Readers without this background will still benefit from the techniques and algorithms discussed.