Part One: Introduction to energetics. What is energetics? Methods for studying energetics. Part Two: Doubly-labelled water: Theory. Biochemical basis of the technique. Major fluxes resulting in the turnover of labels in the body. Assumptions in the model. Other issues. Validation studies in comparison to indirect calorimetry. Part Three: Doubly-labelled water: Practice. Is DLW the best technique to use? Is DLW appropriate for the animal or situation that I wish to study? Legal aspects of applying doubly-labelled water. What effect will DLW have on the subject animal? Calculating the dose to use. Buying isotopes. Field protocols. Laboratory protocols. Calculating CO2 production and energy expenditure. Part Four: Review of estimates of energy expenditure derived from studies using doubly-labelled water: Daily energy demands of wild animals. Specific applications (costs of activity). Domestic animals. Humans. Writing up doubly-labelled water studies.