Operation and Control of Interconnected power system is a field where significant changes are taking place continuously. Energy demand is growing at the rate of nearly ten percent per annum. Generation is added in stages along with transmission capacity. The integration of the five electrical regional grids which operate at different frequencies into a single national grid requires knowledge of various control strategies for proper inter regional exchange of power within the region and within the states. Proper maintenance of power quality is very important. Further given the system in controlled steady sate, economic operation of the power system wherein the generation costs, transmission losses and the related, matters become important. Analysis of the system in operation, development of suitable models for each mode in operation and control and application of mathematical theory to obtain useful results all are important in practice. The aim of this book is to provide at a single place all the relevant information including the latest advanced topics that have emerged in recent years to students of Power Systems at both under Graduate and Post Graduate level.