This book is concerned with dc and ac electric motor drives and the speed and positional control systems that may be devised by using them. The power sources considered are also both dc and ac. Power semiconductor converters of various types are located between motor and source. It will be useful both to practicing engineers and to university students. At the University of Toronto this material is discussed in the final semester of the undergraduate course in electrical engineering. It is fully covered by graduate students in an advanced course.