This synthesis paper describes the HIV epidemics in 15 West African countries - Benin, Burkina Faso, Cote d'Ivoire, Ghana, Guinea, Mali, Niger, Nigeria, Senegal, Togo, Cape Verde, The Gambia, Guinea Bissau, Liberia, and Sierra Leone. It focuses particularly on the degree to which the epidemics in each country are concentrated or generalized, and the implications for effective prevention. The urgent need for rigorous analytic work to equip countries with a sound, situation-specific analytical base for action motivated this study. The analysis team critically reviewed and interpreted all available surveillance, survey and research data (the amount and quality of which varied considerably by country). For the first ten countries listed, short country visits enabled interviews and more thorough search for data and other information, and the programmatic response to the HIV/AIDS epidemic was also assessed to develop recommendations for improving resource allocation and program effectiveness.