Measuring adherence to medical and behavioral interventions is important to clinicians and researchers since inadequate adherence can reduce the effectiveness of an intervention. Unfortunately, there is no gold standard for measuring adherence across health behaviors. Adherence needs to be defined situationally with parameters of acceptable adherence carefully delineated and appropriate to the health behavior being studied. Additionally, measurement methods must be valid, reliable, and sensitive to change; this paper reviews these criteria. Methods used to measure adherence to dietary interventions include 24-hour recalls, food diaries, and food frequency questionnaires. Direct and indirect calorimetry, doubly labeled water, and a variety of self-report methods can be used to measure adherence in physical activity interventions. Adherence to pharmacological interventions is assessed using self-report methods, biochemical measures, medication counts, and the automated pharmacy database review strategy. The strengths and weaknesses of these methods for measuring adherence to dietary, physical activity, and pharmacological interventions are reviewed. Control Clin Trials 2000;21:188S-194S