Physical activity has been associated with the prevention and control of several medial conditions that are major causes of death and disability in the United States. The criterion-based approach adopted by the US Preventive Services Task Force is used to objectively evaluate the evidence regarding the effectiveness of physical activity counseling for healthy adults as a primary preventive intervention in the clinical setting. This evaluation addresses: (1) the burden of suffering attributable to physical inactivity; (2) the efficacy of physical activity in disease prevention in regard to six medical conditions; and (3) the characteristics of the intervention in terms of simplicity, cost, safety, acceptability, and patient compliance. Based on consideration of the evidence for each of these issues, specific recommendations are made regarding the role of physical activity counseling in routine clinical practice. In addition, practical guidelines are presented to aid clinicians in physical activity counseling.