The most accurate measurement of energy expenditure (EE) in free-living individual subjects is by the doubly-labeled water technique. The minute-by-minute heart rate UH) method, while not yet quite suitable for individuals, will give excellent measurements of EE in small groups of subjects as well as the pattern of EE while wearing the tH recorder. In preschool children, chronic energy deficiency results in decreased activity as a first defense against the decreased growth rate. In marginally malnourished school-age children, the lower levels of EE result from their small size. However, they cannot cope with situations where they have increased activity above ordinary levels.