This text takes an approach to teaching operating systems that aims to be different in several different ways: it presents the basic theory of operating system design and implementation in some depth; and it exposes the student to the internals of operating systems by demonstrating how the mechanisms and algorithms are actually implemented in practice. Throughout the theory is illustrated by one running example, Linux, as implemented on a PC. This system is in widespread use in teaching institutions, and its source code is readily and freely available; as each particular topic is being examined in theory, and in its practical implementation, students are introduced to the related system services. The POSIX interface is used for this. The book should appeal to an instructor who is already facing the challenge of drawing material from many sources. It should also encourage others to take up this approach to the teaching of operating systems. It is aimed particularly at operating system modules in specialist computer systems or computer science courses or computer science majors in general science courses it would also be suitable for students on specialist postgraduate courses. While primarily intended as an academic textbook, it will also be of interest in the non-academic field to users of Linux wishing to know more about it.