Readers can get a deep glimpse of operating system basics!!!!!!!! This chapter describes why OS is required, what is OS, functions of OS, etc. This chapter mainly focuses on evolution of OS on the basis of processing, user and special purpose OS. This chapter also focuses on views of OS i.e. user and system view. It elaborates the concept of system call, events and their types. In the middle of chapter, we will discuss the heart of OS i.e. kernel concept and their types. This chapter focuses on Hardware protection, Design and Implementation of the OS on the basis of different parameters. At the end this chapter we will focus on some case studies of different operating system like android, UNIX, windows etc.