This book describes the phenomena of cyclic activity as they are observed in the Sun and solar-type stars. Although there are many books currently available on the Sun, solar activity, and activity phenomena in stars, this is the only one that links these topics and attempts to define stellar parameters that are conducive to activity in general and cyclic activity in particular and to descibe the mechanisms responsible for the various modes of activity. The first seven chapters review our basic knowledge of the Sun, of the solar activity cycle, and of stellar activity cycles. Chapter 1 also describes some of the terrestrial effects of solar activity and stresses the importance of reliable predictions of the parameters of future cycles. Chapters 8 through 13 describe some of the latest research into the solar cycle, including extended activity cycle and the polar fields reversal. The latest contributions from studies of dynamo theory, of helioseismology, and of chaos theory are also covered. The last chapter discusses the methods currently in use to predict parameters of future cycles and their success (or failure) for the maximum of cycle 22.