In the last few decades, community based participatory research (CBPR) has emerged as an important approach that links environmental health and justice advocates with research institutions to understand and address environmental health problems. CBPR has generally been evaluated for its impact on policy, regulation, and its support of community science. However, there has been less emphasis on assessing the ways in which CBPR (re)shapes and potentially improves the scientific enterprise itself. This commentary focuses on this under-emphasized aspect of CBPR-how it can strengthen science. Using two case studies of environmental health CBPR research-the Northern California Exposure Study, and the San Joaquin Valley Drinking Water Study-we posit that CBPR helps improve the "3 R's"of science-rigor, relevance and reach-and in so doing benefits the scientific enterprise itself.