What are the biological differences between men and women and how do they come about? Beyond the obvious reproductive differences, do other biological differences influence the behavior of women and men? For example, not only do women and men differ in various sex hormone levels, but their brains are influenced in different ways by those hormones. To what extent are the differences in sexual behavior among men and women due to genetic variation? How has our evolutionary history influenced our sexuality? Why are cultural sanctions against sexual “cheating” more severe for women than men? Can we make inferences about our own sexuality by examining mating systems in other primates (particularly the great apes)? Our discussion of these and other questions will be facilitated by a careful reading of selected evolutionary, medical, developmental, neurophysiological, and sociobiological literature.