The birds and the bees are much touted for telling us a thing or two about sex. The reality is that, although we can describe pretty well how it happens, for the past 100 years evolutionary biologists have been struggling to explain why sex exists.

At the crux of the matter is the long-standing puzzle that asexual organisms should fare much better than their sexual counterparts — a species of parthenogenetic females, for example, could reproduce twice as fast and wouldn't have to waste valuable time seeking an appealing mate. However, we know from nature that sexually reproducing species predominate, whereas asexual lineages are few and short-lived. So, sex survives despite its obvious costs. Why? And where should we start looking for answers to this 'paradox of sex'? This issue of Nature Reviews Genetics is dedicated to these and related questions, with an emphasis on empirical studies that have become possible with the application of molecular genetic methods to evolutionary issues. These studies are beginning to allow researchers to discriminate between the different theories that account for the evolution of sex.

The study of sex engages scientists from different areas of biology, who investigate how and why sex evolved in the first place, why it is maintained, and how the distinct morphology of the two sexes has developed and evolved in animals and plants. The diversity of these disciplines is reflected in a Web Focus at http://www.nature.com/nrg/focus/evolsex. In this Web Focus, we have grouped the articles published in this issue with related reviews that have appeared in this journal over the past 18 months, and with relevant research articles published by other journals from the Nature Publishing Group. The problem of sex may be an old one, but the variety of issues it raises will continue to provide challenges for years to come.