The article discusses the importance of process evaluation in randomized controlled trials (RCTs) of complex health interventions, particularly in the context of school-based health promotion. The authors argue that while RCTs are the most rigorous method for evaluating intervention effectiveness, they often focus solely on outcomes without considering the processes involved in implementing the intervention. They propose a framework for integrating process evaluation into RCTs, using the RIPPLE study as a case example. The RIPPLE study, a cluster RCT, compared peer-led and teacher-led sex education programs in 27 English secondary schools. The process evaluation aimed to document implementation, describe and compare processes, collect participant experiences, and gather data on school contexts. The results showed that peer-led sex education improved knowledge outcomes, increased satisfaction, and reduced risky sexual behavior among girls, but had no significant impact on other behavioral outcomes. The authors emphasize the need to integrate process and outcome data to maximize the interpretability of results and to understand the conditions under which interventions are most effective. They also highlight the importance of process evaluations in explaining outcome findings and in convincing skeptics about the value of RCTs for complex interventions.The article discusses the importance of process evaluation in randomized controlled trials (RCTs) of complex health interventions, particularly in the context of school-based health promotion. The authors argue that while RCTs are the most rigorous method for evaluating intervention effectiveness, they often focus solely on outcomes without considering the processes involved in implementing the intervention. They propose a framework for integrating process evaluation into RCTs, using the RIPPLE study as a case example. The RIPPLE study, a cluster RCT, compared peer-led and teacher-led sex education programs in 27 English secondary schools. The process evaluation aimed to document implementation, describe and compare processes, collect participant experiences, and gather data on school contexts. The results showed that peer-led sex education improved knowledge outcomes, increased satisfaction, and reduced risky sexual behavior among girls, but had no significant impact on other behavioral outcomes. The authors emphasize the need to integrate process and outcome data to maximize the interpretability of results and to understand the conditions under which interventions are most effective. They also highlight the importance of process evaluations in explaining outcome findings and in convincing skeptics about the value of RCTs for complex interventions.