Given substantial evidence that healthy lifestyle behaviors lessen the odds of cardiovascular disease, a guideline from the American Heart Association and American College of Cardiology1 advises physicians to foster patients’ physical activity. But how is the clinician to evaluate a patient’s healthy lifestyle behaviors, let alone enhance them? Traditionally, patient self-reports supplied almost all behavioral data available to health professionals. However, whether given by free recall, structured questionnaire, or written logs, post hoc surveys inherently manifest forms of error well known to behavioral scientists. People forget. Many have no idea what moderate to vigorous activity feels like. Individuals also experience demands and motivations that distort what they report.