This article discusses how process indicators can complement outcomes as part of a comprehensive explanatory evaluation framework, using the example of skills-based behavioural interventions to prevent sexually transmitted infections and promote sexual health among young people in schools. A systematic review was conducted, yielding 12 eligible outcome evaluations, 9 of which included a process evaluation. There were few statistically significant effects in terms of changes in sexual behaviour outcomes, but statistically significant effects were more common for knowledge and self-efficacy. Synthesis of the findings of the process evaluations identified a range of factors that might explain outcomes, and these were organized into two overarching categories: the implementation of interventions, and student engagement and intervention acceptability. Factors which supported implementation and engagement and acceptability included good quality teacher training, involvement and motivation of key school stakeholders and relevance and appeal to young people. Factors which had a negative impact included teachers' failure to comprehend the theoretical basis for behaviour change, school logistical problems and omission of topics that young people considered important. It is recommended that process indicators such as these be assessed in future evaluations of school-based sexual health behavioural interventions, as part of a logic model.