In the first section, ac and dc technologies are compared highlighting their advantages and disadvantages. Since ac and dc systems have both evolved significantly since their introduction in the mid and latter parts of the 19th century, many of the early advantages of ac systems no longer exist or are of less importance today. Consequently, it is useful to provide a brief historical perspective on the evolution of both ac and dc power systems. As in the dc case, there are many potential modes of operation and control strategies for the given system. In ac systems, the situation is more complex since it is necessary to regulate both the amplitude and the frequency of the ac voltage. In the third section, the techniques of controlling and analyzing the stability of ac systems is reviewed.