It is a pleasant endeavour to bring out this textbook, viz., “CONCEPT OF FACTS” for Anna University affiliated college students of Electrical and Electronics Engineering. Each unit of this book has been prepared in a lucid and comprehensible language with neat diagrams. It will help the students to know the concepts of FACTS controllers in such a way, as if a teacher is in front of them discussing the subject in an easy way. The ultimate aim of this book is to cover the Anna University affiliated-college-syllabus. We are thankful to all the authors and publishers of the other books referred to in this book.