Participatory research (PR) experts believe that increased community and stakeholder participation in research augments program pertinence, quality, outcome, sustainability, uptake, and transferability. There is, however, a dearth of assessments and measurement tools to demonstrate the contribution of participation in health research and interventions. One systematic review of PR, conducted for the Agency for Health Research and Quality (AHRQ), provided no conclusive evidence concerning the benefits of community participation to enhance research and health outcomes. To overcome methodological gaps and barriers of the AHRQ review, we propose to conduct a systematic realist review, which can be understood as a theory-driven qualitative review capable of capturing the often complex, diffuse and obtuse evidence concerning participation. Reviewing how PR mechanisms and contextual factors mediate and moderate outcomes, the review will generate and test hypotheses (middle-range theories) conceptualizing the benefits of participation and will portray the manner and circumstances in which participation influences outcomes.