UNESCO published the International Technical Guidance on Sexuality Education (ITGSE) in 2009. In 2016, they sought an external consultant to update its content to reflect the evidence and lessons learned since the original publication. Two independent researchers undertook a review of the evidence, seeking the highest-quality evidence on the effectiveness and implementation of CSE programmes worldwide since 2008, based largely on selected systematic reviews and large-scale randomized controlled evaluations of school-based sexuality education programmes. While this review emphasizes evidence from rigorous research, it also shares evidence from implementation and practice based on input from UNESCO and an expert Advisory Group, participants at a consensus meeting on sexuality education in October 2016, and the results of an expert survey.