To economize the operating cost of the combined operation of hydal and thermal power plants, operating powers of the hydal and thermal units can be calculated by using different numerical methods and meta heuristic optimization algorithms. This book presents the implementations and comparisons of Particle Swarm Optimization algorithm and its variants like FIPSO and accelerated PSO on the short term hydrothermal scheduling problem. Readers interested in economic dispatch problem solving by using heuristic techniques, are welcome to read the book.