Numerous methods can be used to assess energy expenditure (EE). The Fick method allows an easy calculation of EE in critically ill patients with Swan-Ganz catheters. Indirect calorimetry can be performed using either closed circuit or open circuit techniques. The latter is accurate when the patient is breathing air or low-FiO2 gaz mixture, but inaccurate at high FiO2. The accuracy of instruments, the presence of a leak-free system and the mathematical manipulations are critical points. Calorimetric studies performed in patients receiving high FiO2 must be interpreted with caution.