Population growth will be a major force shaping the next half century with 99% of this growth happening in developing world. 90% in the poorest countries. Today’s 6.7 billion people will have expanded by 2050 to between 7.8 to 10.8 billion; the UN’s medium level projection for 2050 is now 9.2 billion – the number recently increased because of stalled fertility decline in many fast-growing countries. This situation presents major challenges for economic and social development, health, natural resources, and social conflict. To address these issues, Berkeley’s Bixby Center for Population, Health and Sustainability hosted, in collaboration with the Bixby centers at UCLA and UCSF, an international forum titled The World in 2050: A Scientific Investigation of the Impact of Global Population Changes on a Divided Planet. In addition to the implications of the projected population growth, the participating experts from the US, Africa, Asia and Europe recognized the vast and well documented unmet need for voluntary family planning in all of the countries with high average family size and rapid population growth. The papers presented at the forum will be published as a special issue of the Philosophical Transactions B (Biological Sciences), a publication of the Royal Society, London with a forward by Malcolm Potts, Bixby Professor in the School of Public Health at Berkeley, and Professor Roger Short of the University of Melbourne, Australia.