In the past few decades the distribution systems(DS) were facing several problems like high losses & poor voltage regulation. In this context, Distributed Generation turns out to be reliable in eliminating the problems of DS. Determining proper size and location of DG sources in DS is important. Studies have indicated that inappropriate selection of the location and size of DG leads to greater system losses. The book opens with an existing load flow method to determine losses. Then an analytical and PSO methods are developed for the optimal placement of Single DG. The effect of time varying loads and load modeling on location and size of DG is investigated. A new two stage methodology is proposed to determine optimal placement of multiple DG units. In the first stage an Analytical approach based on minimization of real power loss and Fuzzy approach based on minimization of real power loss and improved voltage magnitudes are proposed to determine the locations of DG. In the second stage, GA&PSO are developed to determine the optimal sizes at these locations. This book is useful for professionals in power utilities, research scholars and students of advanced courses in power systems.