The doubly labelled water method uses the principles of indirect calorimetry to measure total energy expenditure from the turnover rates of two stable isotopes: deuterium and oxygen 18. Labelling total body water also provides estimates of body composition and measurements of water outflow rates. Although the principle of the method was determined in the 1950s, it was only applied to humans in the 1980s. Some 15 years later, it is time for an objective appraisal of the method. This review first describes the principle and practice of the doubly labelled water method. The original concept described by Lifson and MacClintock is then discussed, and proposals are made to adapt the method to physiological and pathophysiological situations.