In 1976 I wrote a monograph on lysosomes (Lysosomes: A Survey, Springer- Verlag, Vienna) that was intended as an up-to-date, comprehensive survey. Whatever success I may have achieved then in fulfilling that intention, even the effort now would be foolhardy. The literature has grown so rapidly in the past decade that I certainly could not even read all of the essential papers, let alone understand and analyze them. My goal here, therefore, is simply to introduce the major features of lysosomes at a level I hope will be useful both to I;ldvanced students and to researchers interested in obtaining a broad background. This is in keeping with the design of the Cellular Organelles series: the series is more a set of advanced texts than of review monographs. This design carries with it the decision not to support each point by refer- ences to the original literature. I apologize for the injustice involved in such a decision but feel that in any event it would be impossibly unwieldy to cite, adequately and in a balanced manner, the contributions of the vast network of researchers responsible for the information upon which I draw.