Due to the inductive nature of most of the electric power system components and loads, reactive power has an important role in the transmission and distribution of electric energy with desired quality. Power transmission capability and voltage regulation and stability are important issues in modern networks. In this course, the uncompensated lines and loads characteristics and the effects of reactive compensation on them are analyzed. Then, various types of fixed and controllable compensators are introduced, and principles of reactive power compensation are presented. Finally, voltage instability phenomena, analysis methods, and countermeasures are discussed.