The article discusses the importance of process evaluation in randomised controlled trials (RCTs) of complex interventions, using the RIPPLE study as an example. Complex interventions are those that are not drugs or surgical procedures but have multiple components. RCTs are the most rigorous way to evaluate interventions, but complex interventions require adaptations to the standard design of such trials. The RIPPLE study, a cluster RCT, aimed to investigate whether peer-led sex education was more effective than teacher-led sessions in reducing risky sexual behaviour. It involved 27 English secondary schools and followed participants up to age 19. The study included a process evaluation to document the implementation of the peer-led intervention and compare it with teacher-led education. The process evaluation used various methods, including questionnaires, focus groups, interviews, and observations. The outcome results showed that the peer-led approach improved some knowledge outcomes, increased satisfaction with sex education, and reduced intercourse in girls. However, it did not significantly affect other behavioural outcomes. The process data were analysed to answer questions about the relationship between trial outcomes and the implementation of the intervention. The analysis revealed that the peer-led approach was more effective when the sex education was participative and skills-based. However, it was less effective in engaging students at higher risk of poor sexual health. The article argues that integrating process and outcome evaluations is crucial for improving the science of RCTs. It highlights the importance of collecting both qualitative and quantitative data and analysing process data before outcome data to avoid bias. The article also discusses the need for integral process evaluations in trials of complex interventions to better understand the effectiveness of interventions and their implementation. The lessons from the natalizumab case highlight the risks of fast-tracking new drugs without sufficient safety data. The article concludes that process evaluations are essential for enhancing the explanatory power and generalisability of RCTs.The article discusses the importance of process evaluation in randomised controlled trials (RCTs) of complex interventions, using the RIPPLE study as an example. Complex interventions are those that are not drugs or surgical procedures but have multiple components. RCTs are the most rigorous way to evaluate interventions, but complex interventions require adaptations to the standard design of such trials. The RIPPLE study, a cluster RCT, aimed to investigate whether peer-led sex education was more effective than teacher-led sessions in reducing risky sexual behaviour. It involved 27 English secondary schools and followed participants up to age 19. The study included a process evaluation to document the implementation of the peer-led intervention and compare it with teacher-led education. The process evaluation used various methods, including questionnaires, focus groups, interviews, and observations. The outcome results showed that the peer-led approach improved some knowledge outcomes, increased satisfaction with sex education, and reduced intercourse in girls. However, it did not significantly affect other behavioural outcomes. The process data were analysed to answer questions about the relationship between trial outcomes and the implementation of the intervention. The analysis revealed that the peer-led approach was more effective when the sex education was participative and skills-based. However, it was less effective in engaging students at higher risk of poor sexual health. The article argues that integrating process and outcome evaluations is crucial for improving the science of RCTs. It highlights the importance of collecting both qualitative and quantitative data and analysing process data before outcome data to avoid bias. The article also discusses the need for integral process evaluations in trials of complex interventions to better understand the effectiveness of interventions and their implementation. The lessons from the natalizumab case highlight the risks of fast-tracking new drugs without sufficient safety data. The article concludes that process evaluations are essential for enhancing the explanatory power and generalisability of RCTs.