Propagation is the ultimate purpose of any species, making human reproduction, specifically fertility and its scientific study, a subject uniquely intimate and relevant to us all. With the total world population now in excess of 6 billion and growing at a rate of some 75 million per year, boosting fertility would seem to be a moot topic. However, for the millions of couples facing infertility problems and the hundreds of thousands of couples attempting to conceive through assisted reproductive technologies annually, the science of fertility is key to their quest for genetically related children.

While the wishes of hopeful couples drive developments in the clinic, advances in understanding the basic science underlying fertility point the way towards future innovations. But many of the resulting manipulations and technologies have raised moral, ethical and judicial questions that societies and governments are often inadequately prepared to face.

It is against this backdrop of basic investigations and clinical application, combined with matters both personal and public, that we at Nature Cell Biology and Nature Medicine present this joint Supplement on Fertility. We have brought together the cellular, biological, medical and ethical aspects of fertility, and the result is a comprehensive perspective of current developments in the field. Areas covered include the science of gamete production and fertilization, genetic and environmental influences on fertility and contraception. Current infertility treatments and the economics, ethics and safety of their application are also discussed. It is our hope that these perspectives will not only inform, but also bring fresh insight to the current debates over infertility treatments and their applications.

The accompanying website, http://www.nature.com/fertility, also features selected papers in the field of fertility from Nature Cell Biology, Nature Medicine and Nature, and is free to all visitors for six months. As a joint project of Nature Cell Biology and Nature Medicine, articles within this supplement should be cited as page number Sxx�Sxx of supplement S1, either as volume 4 of Nature Cell Biology or volume 8 of Nature Medicine.

We are delighted to acknowledge the financial support of the World Health Organization, the National Institutes of Child Health and Human Development and of Environmental Health Sciences, and CONRAD in producing this supplement. However, Nature Cell Biology and Nature Medicine retain sole responsibility for editorial content and its peer review.