This book is the result of a serendipitous meeting between Rob Foley and myself in the palaeontology collections of the National Museums of Kenya in July, 1987. Rob was introduced to me one afternoon while I was measuring skulls of Theropithecus oswaldi. He too was visiting to look at fossil Theropithecus and we soon struck up a conversation about our mutual fossil friends. Over the course of that afternoon and the next few days we discussed many aspects of the evolution and ecology of fossil and living Theropithecus. We quickly realized that the last twenty-five years had witnessed a dramatic increase in our knowledge of biology of the genus, ranging from behavioural and ecological information on the extant species, Theropithecus gelada, to information drawn from molecular biology, karyology, and palaeontology bearing on the evolutionary relationships of the genus. What could we piece together of the history and palaeoecology of the genus from the relatively large samples of fossil materials of the various extinct species available for study? What could this body of information tell us about the evolution of Theropithecus itself, and what general lessons about speciation, rates of evolution, extinction, and other phenomena might this information hold for all students of primate and human evolution? Realizing that many of the people instrumental in bringing this evidence to light were still very much alive and taking an active interest in Theropithecus biology, we decided that a meeting of these individuals would be timely and appropriate. Thus, the idea for the symposium ‘Theropithecus as a casestudy in primate evolutionary biology’ was born.