This paper focuses on a review of studies assessing the effectiveness of sex education programmes in reducing risky sexual behaviour among adolescents and young people. The paper concludes that sex education programmes that are based on a comprehensive curriculum can delay the onset of sexual activity among adolescents and young people, reduce the frequency of intercourse, reduce the frequency of unprotected sex, reduce the number of sexual partners, and increase condom and contraceptive use. Furthermore, sex education programmes do not increase sexual activity among adolescents and young people and generally result in increased knowledge about human sexuality. To be effective, sex education programmes need to provide unbiased and scientifically based information; explain the various practices and methods that can reduce the risks of pregnancy and of acquiring a sexually transmitted infection during intercourse; discuss values, perceptions and norms; and use methods validated by pedagogy and behavioural-change science to build the skills that adolescents and young people need in order to make responsible decisions about their sexual behaviour and follow through with those decisions.