We explored constraints of implementing AIDS education in public schools in Kenya. Sixty interviews with teachers and 60 focus group discussions with students were conducted in 21 primary and nine secondary schools. System/school-level constraints included lack of time in the curriculum, limited reach of secondary-school students (because AIDS education is embedded in biology, which is not compulsory), and disapproval of openness about sex and condoms by the Ministry of Education and parents. Alternative strategies to teach about AIDS had their own constraints. Teachers lacked training and support and felt uncomfortable with the topic. They were not used to interactive teaching methods and sometimes breached confidentiality. Teachers' negative attitudes constrained students from seeking information. Training interventions should be provided to teachers to increase their self-confidence, foster more positive attitudes, and stimulate interactive teaching methods. The Ministry of Education needs to have a clear policy toward the promotion of condoms.