Counseling by health care providers has the potential to increase physical activity in sedentary patients, yet few studies have tested interventions for physical activity counseling delivered in health care settings. The Activity Counseling Trial (ACT) is a 5-yr randomized clinical trial to evaluate the efficacy of two primary care, practice-based physical activity behavioral interventions relative to a standard care control condition. A total of 874 sedentary men and women, 35-75 yr of age, have been recruited from primary care physician offices at three clinical centers for 2 yr of participation. They were randomly assigned to one of three experimental conditions that vary, in a hierarchical fashion, by level of counseling intensity and resource requirements. The interventions, which are based on social cognitive theory and the transtheoretical model, are designed to alter empirically based psychosocial mediators that are known to be associated with physical activity. The present paper describes the theoretical background of the intervention, the intervention methods, and intervention training and quality control procedures.