The objective of this review is to summarize recent advances in energy requirements in humans. A surge of information has been made possible over the last several years due to the ability to measure free-living energy expenditure in unrestricted subjects with the doubly labeled water technique. One of the major applications of this new technique is to establish data on energy requirements based on energy utilization. We shall review data in healthy humans across the life-span and in addition review the effects of other factors such as disease, obesity, pregnancy and exercise, which are known to effect energy requirements.