OBJECTIVES: Sex and relationship education (SRE) is regarded as vital to improving young people's sexual health, but a third of schools in England lacks good SRE and government guidance is outdated. We aimed to identify what makes SRE programmes effective, acceptable, sustainable and capable of faithful implementation. DESIGN: This is a synthesis of findings from five research packages that we conducted (practitioner interviews, case study investigation, National Survey of Sexual Attitudes and Lifestyles, review of reviews and qualitative synthesis). We also gained feedback on our research from stakeholder consultations. SETTINGS: Primary research and stakeholder consultations were conducted in the UK. Secondary research draws on studies worldwide. RESULTS: Our findings indicate that school-based SRE and school-linked sexual health services can be effective at improving sexual health. We found professional consensus that good programmes start in primary school. Professionals and young people agreed that good programmes are age-appropriate, interactive and take place in a safe environment. Some young women reported preferring single-sex classes, but young men appeared to want mixed classes. Young people and professionals agreed that SRE should take a 'life skills' approach and not focus on abstinence. Young people advocated a 'sex-positive' approach but reported this was lacking. Young people and professionals agreed that SRE should discuss risks, but young people indicated that approaches to risk need revising. Professionals felt teachers should be involved in SRE delivery, but many young people reported disliking having their teachers deliver SRE and we found that key messages could become lost when interpreted by teachers. The divergence between young people and professionals was echoed by stakeholders. We developed criteria for best practice based on the evidence. CONCLUSIONS: We identified key features of effective and acceptable SRE. Our best practice criteria can be used to evaluate existing programmes, contribute to the development of new programmes and inform consultations around statutory SRE.