This publication aims to document the experience of the world's largest international HIV/AIDS prevention project, which was implemented by FHI (Family Health International) and its partners in 45 countries. It describes lessons learned during AIDSCAP, with examples and project profiles, in 10 technical and programmatic areas: behaviour change communication, improving prevention and treatment of sexually transmitted diseases, prevention marketing, policy development, behavioural research, evaluation, gender and HIV/AIDS, management, AIDS care and support, and cross-border interventions.