Operating System, an integral part of any computer, is the interface between the computer users and the hardware. This comprehensive book provides the readers with the basic under-standing of the theoretical and practical aspects of operating systems. The text explains the operating systems and components of operating systems including attributes of Linux and Unix operating systems. It also discusses Android operating system and Tablet computer. The book explicates in-depth the concepts of process, threads/multithreading and scheduling and describes process synchronization, deadlocks and memory management including file access methods and directory structure. In addition, it also describes security and protection along with distributed file systems. The book is designed as a textbook for undergraduate students of Electronics and Communication Engineering, Computer Science and Engineering and Information Technology as well as post-graduate students of computer applications and computer science.