Because critical illness often creates a vigorous metabolic response to permit the repair of injured tissues, nutritional considerations are essential in the medical management of the critically ill patient. Individuals with seemingly adequate endogenous nutritional reserves may rapidly develop complications of starvation. Nutritional supplementation is essential; however, critically ill patients may not readily tolerate nutritional support. Calories may need to be withheld until the patient is able to tolerate and utilize nutritional support. Once the decision to initiate nutritional support is made, nutritional status, level of stress, metabolic condition, and vital organ function influence the patient's nutritional requirements. An assessment of the patient's metabolic condition provides data useful in determining the need for supplemental electrolytes or macronutrients. These data also provide information regarding vital organ function, which is necessary for utilization of the fuel and substrate. Aggressive monitoring and judicious nutritional supplementation will afford the critically ill patient the best chance of recovery.