This book consists of five chapters, which are organized as, chapter I, which briefly introduces the background and objectives of this book. Chapter II describes concepts of voltage sag, voltage sag location method are discussed and reviewed. Chapter III presents a new method for identifying the exact voltage sag source locations in power systems by using the multivariable regression (MVR) model. In this method, are considered the training data for each system bus, estimate all the unmonitored bus voltages, calculated the VD and SD indices for locating the exact voltage sag. Chapter IV presents the results of voltage sag source exact location identification for the IEEE 9 Bus, 30 Bus and modified 69 Bus test systems. In chapter V, the major achievements of the book are summarized.