In this chapter, we discuss the essential C programming concepts and constructs for implementing C applications. You will be revising the importance of procedure-oriented programming and practicing C functions. You will start with practicing the C functions, recursive functions, and nested functions programming in terms of performance improvement. Mainly, you will be learning C functions' runtime execution environments in detail to handle the performance and security issues at the system level. For instance, it is necessary to learn the details of the runtime memory layout and execution procedure of a program to protect it, optimally manage its resources, and improve the code execution speed. You will be doing related hands-on activities to easily grasp the C functions-related concepts and developing C functions to avoid performance and security issues.