Is there a sufficient evidence base for the U.S. Department of Health and Human Services (HHS) to develop a comprehensive set of physical activity guidelines for Americans? To address this question, the Institute of Medicine (IOM) held a workshop titled "Adequacy of Evidence for Physical Activity Guidelines Development" in Washington, DC on October 23-24, 2006, sponsored by HHS.The workshop summary includes the presentations and discussions of more than 30 experts who were asked to consider the available evidence related to physical activity and the general population, as well as special population subgroups including children and adolescents, pregnant and postpartum women, older adults, and persons with disabilities. The summary provides an overview of the specific issues of relevance in assessing the quality and breadth of the available evidence.