This is a detailed introduction to operating systems which is aimed particularly at students on HNC, HND and degree courses in computer studies and computer sciences on which operating systems is an essential subject. It is also suitable for professionals and computer managers who are responsible for computer maintenance and operating systems, but are not involved technically. An elementary prior knowledge of hardware and computer programming is assumed, and the concepts and terminology are introduced at the beginning, before understanding is developed on the knowledge gained. Technical content is blended with real-practice issues, and a wide range of exercises is included.