Process evaluation is an important, but infrequently conducted, component of evaluating the impact of health promotion interventions. The process evaluation results from the nine 5-a-Day projects were overviewed. Process evaluation helped explain some of the weaker aspects of program performance, process indicators occasionally declined over time and varied by demographic characteristics, and some process measures were related to mediating variables and program outcomes. Future development of process evaluation must include further development of concepts, more consistent and thorough conduct of process evaluation, appropriate methodological work, and assessment of the relations among the process evaluation components and the program mediators and outcomes. Further development in this area promises refinement in understanding how and why interventions achieve their effects, how best to conduct intervention programs to maximize effects, and enhancement of the internal and external validity of the studies.