Operating Systems are an essential part of any computer system. Similarly, a course on operating systems is an essential part of any computer science education. I wrote this book as a text for an introductory course in operating systems at the junior or senior undergraduate level or at the first-year graduate level. We hope that practitioners will also find it useful. It provides a clear description of the Concepts that underlie operating systems. Concepts are presented using spontaneous descriptions. The fundamental concepts and algorithms covered in the book are often based on those used in both commercial and open-source operating systems. My aim is to present these concepts and algorithms in a general setting that is not tied to one particular operating system. However, we present a large number of examples that pertain to the most popular and the most innovative Operating Systems.