In recent times, Cyber-Physical Systems (CPSs) have become an emerging paradigm for controlling and managing the ever-growing number of cyber-connected devices and has seen an increased use in different applications. Among the various benefits that CPS provides leading to its wide adaptation, one of them is its ability to seamlessly integrate systems from cyber and physical domain to create value. However, to fully realize such benefits various complementary aspects need to be considered. One such aspect is security, which secures the communication between different devices and ensures the prevention of unauthorized access. This chapter sets the base for the rest of the book by discussing the structure of the CPS and its different associated components. It then discusses some of the various different attacks from which CPS need to be secured and the challenges involved in addressing them.