This chapter presents the state estimation of the power system. The chapter starts with General State Estimation Definition and Functions. Energy Management System and Importance of State Estimators in Power Systems are discussed. The chapter includes the Methods of State Estimation as Weighted Least Square algorithms, error characteristics, Newton iteration, bad data detection, Supervisory Control, Data Acquisition, and Phasor Measurement Units. Techniques of State Estimation for Non-Linear Systems are described. Non-Linear Kalman Filter, The Extended Kalman Filter, The Unscented Kalman Filter Methods are presented.