The electric power system industry is undergoing massive changes around the world. Despite the changes with different structures, market rules, and uncertainties, an energy management system (EMS) control centre must always be in place to maintain the security, reliability, and quality of electric service. The function of state estimation is reviewed in this book and is becoming more important, because it is the primary tool for monitoring and control based on real-time data received from measurement units. Also discussed in this book, are the design and application criteria for a proposed over-current relay in radial distribution networks; load modelling in power systems; wide-area stability and voltage control of large interconnected power systems and others.