The U.S. Department of Health and Human Services' "2008 Physical Activity Guidelines for Americans" represents a major milestone in public health efforts to address inactivity. These comprehensive federal physical activity guidelines affirm the strong scientific evidence for the health benefits of regular physical activity. The document contains key guidelines for children and adolescents, adults, older adults, women during pregnancy and the postpartum period, adults with disabilities, and people with chronic medical conditions. There are also key guidelines for injury prevention. The 2008 guidelines provide benchmarks for assessing the health burden of inactivity and inform the design of public health initiatives to promote physical activity in Americans. This article provides a brief review of the evolution of guidelines in the United States, the process for developing the guidelines, and the key recommendations of the guidelines.