The emerging technology of Flexible AC Transmission System (FACTS) enables planning and operation of power systems at minimum costs, without compromising security. This is based on modern high power electronic systems that provide fast controllability to ensure 'flexible' operation under changing system conditions. This book presents a comprehensive treatment of the subject by discussing the operating principles, mathematical models, control design and issues that affect the applications. The concepts are explained often with illustrative examples and case studies. In particular, the book presents an in-depth coverage of: Applications of SVC, TCSC, GCSC, SPST, STATCOM, SSSC, UPFC, IPFC and IPC for voltage/power control in transmission systems; Application of DSTATCOM, DVR and UPQC for improving power quality in distribution systems; Design of Power Oscillation Damping (POD) controllers; Discrete control of FACTS for improving transient stability; Mitigation of SSR using series FACTS Controllers; Issues affecting control design such as electromagnetic and harmonic interactions. The book can serve as a text or reference for a course on FACTS Controllers. It will also benefit researchers and practicing engineers who wish to understand and apply FACTS technology.