&NA;The metabolic response to critical illness and injury increases the metabolic rate and increases mobilization of amino acids from the peripheral tissues. This is done through a neuroendocrine response with elevated levels of catecholamines, glucocorticoids, inflammatory cytokines, and other products of inflammation. Control of the injury, restoration of hemodynamic stability, and early nutrition can minimize the drain on the lean body mass and improve the chance of survival. In this article, the authors summarize the metabolic response to stress and injury