Costing and cost-effectiveness data for HIV prevention programmes are important tools for decision-makers. In many countries, HIV prevention efforts for young people have increasingly focused on schools, with many ministries of education in the process of scaling up school-based sexuality education programmes. However, most ministries of education are implementing programmes without an adequate understanding of how much the programmes cost per learner or per HIV/STIs or unintended pregnancy averted. Costing and cost-effectiveness studies can therefore assist governments and other programme developers and implementers to advocate for and better plan feasible and sustainable programme scale-up. In response to this need for a better evidence base, in 2010 UNESCO commissioned a six-country study into the cost of good quality sexuality education programmes in a sample of low-, middle- and high-income countries. The objective was to develop tools for measuring the cost and cost-effectiveness of HIV prevention programmes and to determine the cost of scaling up good quality HIV and sexuality education in schools. The cost-effectiveness component also measured the projected impact these programmes can have on reducing rates of HIV infection, other STIs and unintended pregnancy.