At the present time, power systems are forced to operate at almost full capacity. Generation patterns result in heavy flows that tend to incur greater losses as well as threatening stability and security of the system. The utilization of the existing power system can be improved through the application of advanced power electronics technologies. Flexible AC Transmission Systems (FACTS) provide technical solutions to address the new operating challenges being presented today. This book presents a complete formulation of the problem of optimizing power system performance using shunt and series FACTS devices considering power system''s reliability, steady state performance, and stability under small and large perturbations. The optimization process is performed using a particle swarm optimization (PSO)- based algorithm capable of pursuing global minimization with desired accuracy and reasonable computational time. This study should be useful to anyone interested in how to formulate the problem of controlling the power system performance, how to address operating limits and stability constraints, and how to choose between different optimization approaches.