Background: Evaluation studies examining the effectiveness of interventions tend to offer little insight into the mechanisms responsible for changes in outcomes. The present study has conducted a thorough process evaluation alongside a longitudinal quasi-experimental trial investigating the effects of a home visitation program for the early detection of health problems among potentially frail community-dwelling older people (? 75 years). We aim to describe the rationale for and steps undertaken in developing a process evaluation plan to identify the factors that influence the success or failure of this complex, patient-centered intervention within the primary care setting.Method: Using a theoretical framework underlying the process evaluation, process evaluation questions are formulated per component of the framework (i.e., implementation fidelity, dose delivered, dose received, reach, recruitment and context). The process evaluation plan shows how both quantitative (e.g., structured registration forms) and qualitative methods (e.g., semi-structured interviews) are applied in gathering process data for a complex, patient-centered intervention integrated within general practices. Process data are gathered with either formative or summative purposes among practice nurses and general practitioners from participating general practices and a purposive sample of older people.Conclusion: Conducting a process evaluation alongside a clinical trial will assist in deciding to what extent the intervention is effective, as well as what factors contribute to the intervention?s effectiveness. The insights gained are imperative for the development of patient-centered interventions that are likely to be sustained when implemented in the intended context.