Adolescents worldwide are at high risk for adverse consequences of sexual activity, including HIV, other sexually transmitted diseases, and unintended pregnancy. Effective intervention strategies are needed to address this risk. This article discusses the advantages of the reasoned action approach for developing such strategies, including the ability to integrate population-specific qualitative information with the approach to develop an intervention that is both theoretically grounded and culturally appropriate. It also describes an application of the approach in developing “Let Us Protect Our Future,” a culturally appropriate HIV risk-reduction intervention for adolescents in South Africa, where sexually transmitted HIV infections are having an especially devastating impact. The results of a randomized controlled trial revealed that grade 6 students in schools that received the intervention were less likely to report having sexual intercourse, unprotected sexual intercourse, and multiple sexual partners during the 12-month follow-up period than their counterparts in control schools.