In most animals, from bees to bison, house flies to humans, it is possible to see at a glance whether an individual is male or female. How and why have these differences in appearance and behaviour developed and what are the nature and extent of the differences between men and women? This book reviews the latest molecular, genetic, hormonal, anatomical and behavioural data in a wide range of species in a series of lively and highly readable articles from the world's leading experts in this field. Such an overview has never before been attempted. It will have a wide appeal, especially to undergraduates and graduates in the biological and medical sciences, and will help to bridge the gap between those who study genes and molecules in the laboratory and those who study the behaviour of animals in the wild.