Start with a Girl: A New Agenda for Global Health sheds light on the realities of girls' health and wellbeing in developing countries, on the links between the health of girls and the prospects for their families, and on the specific actions that will improve health prospects for millions. This report describes the most prevalent and serious health problems adolescent girls face in developing countries, linking them to a combination of specific public-health risks and social determinants of health. It highlights the diverse ways in which governments and non-governmental organizations have sought - often successfully, albeit on small scale - to break vicious cycles of ill health. Finally, and most importantly, the report lays out an ambitious yet feasible agenda for governments, donors, the private sector, and civil society organizations - complete with estimates of indicative costs.