An operating system is an essential component of computers, laptops, smartphones and any other devices that manages the computer hardware. This book is a complete textbook that includes theory, implementation, case studies, a lot of review questions, questions from GATE and some smart tips. Many examples and diagrams are given in the book to explain the concepts. It will help increase the readability and understand the concepts.The book is divided into 11 chapters. It describe the basics of an operating system, how it manages the computer hardware, Application Programming interface, compiling, linking, and loading. It talks about how communication takes place between two processes, the different methods of communication, the synchronization between two processes, and modern tools of synchronization. It covers deadlock and various methods to handle deadlock.It also describes the memory and virtual memory organization and management, file system organization and implementation, secondary storage structure, protection and security.