The doubly labeled water technique represents an unobtrusive and noninvasive means to measure total daily energy expenditure in free-living human subjects who are unaware that energy expenditure is being measured. When combined with measurement of resting energy expenditure, the doubly labeled water technique can also be used to estimate energy expenditure related to physical activity. The relatively recent availability of the doubly labeled water technique in humans has led to several advances in the fundamental understanding of whole body energy metabolism in several important areas. The purpose of this paper is to review the areas in which the doubly labeled water technique has specifically advanced our understanding of whole-body energy metabolism in young children.