The purposes of this paper are: to assess how comprehensive sexuality education (CSE) is implemented in schools in the World Health Organisation’s European Region; and to investigate the evidence supporting its effectiveness. Data were collected in 2016–2017, using a validated questionnaire sent to representatives of governmental and nongovernmental institutions in 25 countries of the WHO European Region. The results demonstrated that, in nine countries, sexuality education can be classified as comprehensive; in ten countries it is non-comprehensive; in four countries there is no programme in place; and two countries were excluded from the analysis. In contrast to non-comprehensive sexuality education programmes, CSE programmes address a wider range of topics, including the social, emotional and interpersonal aspects of sexuality. Furthermore, teachers are more often trained to deliver sexuality education and participatory teaching methods are widely used. CSE programmes are more valued by pupils as a source of information on sexuality, based on national survey results. The availability of CSE programmes coincides with more effective contraceptive use and lower teenage fertility rates. However, more rigorous research is needed to establish a causal relationship between CSE and adolescent sexual and reproductive health indicators.