Family planning is about listening to what parents want, not dictating what they should do; it is about enabling women and their partners to decide freely when and how many children they want to have. This Lancet Series reviews the evidence for the effects of population and family planning on people's well-being and the environment. The Series first appeared ahead of the London Summit on Family Planning, hosted by the UK Government, on Wednesday, July 11, 2012. The Summit brought together participants from across the world to mobilise global action supporting the rights of 120 million additional women and girls to access family planning without coercion or discrimination. The Lancet Series amalgamates the latest thinking underpinning these crucial deliberations, showing how lack of access to family planning carries a huge price, not only in terms of women's and children's health and survival but also in economic terms. The Lancet Global Health Series are groups of articles on a topic of global health importance. Series papers and Comments, related articles, links to relevant publications, media highlights, and audio interviews are available from http://www.thelancet.com/global-health-series. Research into prevalent conditions and wider health policy issues are reported by a range of experts.