Meaningful improvements in health require modifying the social determinants of health. As policies are often underlying causes of the living conditions that shape health, policy change becomes a health goal. This focus on policy has led to increasing interest in expanding the focus of community-based participatory research (CBPR) to change not only communities but also policies. To best realize this potential, the relationship between evidence and power in policy change must be more fully explored. Effective action to promote policies that improve population health requires a deeper understanding of the roles of scientific evidence and political power in bringing about policy change; the appropriate scales for policy change, from community to global; and the participatory processes that best acknowledge the interplay between power and evidence.