How do health care providers know if their patients are getting enough physical activity to promote good health and to reduce their risks for chronic diseases and injury? The first step is to identify the patient's current level of physical activity using questionnaires and/or motion sensors. Questionnaires assess activity levels by having patients answer a set of questions about the types and amounts of activity performed at some time in the past. Motion sensors assess physical activity by patients wearing a small monitoring device that records their body movement as it occurs. If a provider is interested in determining a patient's caloric energy expenditure, he/she can apply statistical regression models to the questionnaire and motion sensor data to estimate kilocalories. If more precise measures of energy expenditure are desired, a provider can use the isotopic doubly-labeled water methods to estimate kilocalories, however this method is costly and is impractical in non-research clinical settings.