This book explore the knowledge of the reader to the basic concepts of Operating Systems in line with the syllabi prescribed by the Anna University- Chennai. This book is designed to help the students to understand the subject easily and prepare for the University Examinations. The chapters in the book are clearly understandable for the students in such a way that the concepts are easily mentioned. Review questions are given at the end of each chapter. Review questions are separated as short answer questions and essay type questions. Each chapter is explained with illustrative example problems and diagrammatically represented wherever necessary.