This book deals with the automatic generation control of interconnected thermal systems with Combination of the automatic voltage control and market based deregulated system. In this Particular work thermal unit is considered with two area concept. The primary purpose of the AGC is to balance the total system generation against system load and losses so that the desired Frequency and power interchange with neighboring system are maintained. Any mismatch between generation and demand causes the system frequency to deviate from scheduled value. Thus high frequency deviation may lead to system collapse. Further the role of automatic voltage regulator is to hold terminal voltage magnitude of synchronous generator at a specified level. The interaction between frequency deviation and voltage deviation is analyzed in this book. System performance has been evaluated at various loading disturbances and at the last market structure is considered. Availability of frequency based tariff scheme is employed. Demand can be managed by managing the tariff.