Most books on operating systems are strong on theory and weak on practice. This one aims to provide a better balance between the two. It covers all the fundamental principles in great detail, including processes, interprocess communication, semaphores, monitors, message passing, scheduling algorithms, input/output, deadlocks, device drivers, memory management, paging algorithms, file system design, security, and protection mechanisms. But it also discusses one particular system MINIX 3a UNIX-compatible operating system in detail, and even provides a source code listing for study. This arrangement allows the reader not only to learn the principles, but also to see how they are applied in a real operating system.