Nearly twenty years have passed since the publishing of a broadly-based texthook on hiolog­ ical control (Huffaker and Messinger 1976). In the interim, other works on biological control have been either briefer treatments (DeBach and Rosen 1991), or collections of essays on selected topics (Waage and Greathead 1986; Mackauer and Ehler 1990). Our text has been written to fill what we believe is a need for a well-integrated, broadly-based text of appropriate length and degree of technical detail for teaching a one semester upper level course in hiological control. We have attempted to focus on principles and concepts, rather than on biological control of particular taxa or hiological control by particular kinds of natural enemies. Therefore, for example, the reader will find the material on biological control of weeds integrated with biological control of insects and mites into chapters on principles, techniques, and applications rather than presented separately. Only biological control of plant pathogens is addressed independently, an appraoch made necessary by the many special features and concepts in plant pathogen biological control.