A patient's nutritional support is a critical part of his general health care scheme. It aims to prevent or correct malnutrition, reduce morbidity, length of hospital stay and treatment costs. It equally optimises the patient's convalescence and quality of life. An optimal support requires the nutritional evaluation of the patient which includes determination of the energy expenditure. Once the energy needs have been evaluated, the clinician is then able to treat any nutritional deficiencies. This article therefore looks at the different methods available for predicting and measuring energy expenditure. The clinical limitations of these techniques and particular cases encountered (intensive care unit, anorexia and severe obesity, geriatrics, HIV infection, etc) will also be discussed.