This book will demonstrate the idea of load frequency control. Power system is a complex network where controlling to frequency is essential. This book will give a close picture of how frequency varies with load, why frequency should be kept under control and what will be the effect on the power system if it keeps varying. Different components and their functions of load frequency control are discussed in details. Two types of system: first One area system and second Two area system are considered. Both system configurations are described and their MATLAB simulations are given in book. The use of controller like PID and its parameters optimization using newest meta-heuristic algorithm called firefly algorithm is also discussed. So after reading this book one will know what is load frequency, its importance, different types of systems, how to simulate it in MATLAB and about optimization techniques.