'Gender' is a word that has become part of HIV/AIDS-related vocabulary and conceptual understanding. However, gender and development approaches understand it as denoting only two identities: male and female. Sexuality is likewise assumed to be heterosexuality. This thinking has informed HIV/AIDS prevention strategies worldwide, and the result is to exclude people who differ from these definitions, leaving them at risk of HIV infection. Research in South-East Asia, and specifically Cambodia, reveals the existence of diverse gender and sexual identities. Some innovative work has begun to develop HIV/AIDS prevention activities with individuals whose gender and sexuality defy conventional categories.