The concept of the Metabolic Equivalent (METs) arose from the cardiac rehabilitation field as a way to express the energy cost of human movement. A MET can be expressed as a reflection of the oxygen cost of activity (1 MET = 3.5 ml/kg/min) or as a measure of calorie energy expenditure (1 MET = 1 kcal/kg/hour). In survey research and in rehabilitation settings, METs are used to express the energy cost of specific activities, with a MET level computed as the energy cost of an activity divided by the resting energy expenditure. The Compendium of Physical Activities provides a comprehensive list of MET levels for physical activities performed in various settings. In survey research, MET levels are multiplied by the frequency and duration of physical activities to estimate the energy expended in moderate and vigorous intensity physical activity. In rehabilitation settings, METs are used as a guide for exercise prescription and exercise safety. Keyword(s): assessment, measurement/evaluation, physical activity