With the increasing availability of effective, evidence-based physical activity interventions, widespread diffusion is needed. We examine conceptual foundations for research on dissemination and diffusion of physical activity interventions; describe two school-based program examples; review examples of dissemination and diffusion research on other health behaviors; and examine policies that may accelerate the diffusion process. Lack of dissemination and diffusion evaluation research and policy advocacy is one of the factors limiting the impact of evidence-based physical activity interventions on public health. There is the need to collaborate with policy experts from other fields to improve the interdisciplinary science base for dissemination and diffusion. The promise of widespread adoption of evidence-based physical activity interventions to improve public health is sufficient to justify devotion of substantial resources to the relevant research on dissemination and diffusion.