Reactive power compensation has been widely used to increase the steady state transmittable power by controlling the voltage profile along the transmission lines in power systems. A number of reactive compensation devices such as shunt capacitor, static Var compensator (SVC) and static synchronous compensator (STATCOM) have long been applied by electric power utilities for this purpose. However, the benefits of reactive power compensation depend greatly on the placement and size of the compensators. In this book, heuristic optimization techniques are employed to determine the optimal location and size of Var compensators such as shunt capacitor, SVC and STATCOM in transmission networks to enhance voltage stability and improve voltage profile by minimizing power loss and total cost. A comparative study has also been done to evaluate the effectiveness of the different Var compensators.