This text provides an elementary level discussion of recent theory relating to the evolutionary and adaptive aspects of reproductive behaviour. The relation between ultimate and proximate levels of explanation is the major theme of the book. Two new chapters in this edition incorporate findings from recent research and there is also new material on humans, physiology, and development. Sex and reproductive behaviour are examined from an evolutionary comparative perspective and numerous empirical studies and examples are cited. This book should be of interest to for students on introductory psychology courses.