Critically ill (intensive care unit [ICU]) patients are characterized by organ failure, intense inflammatory response, insulin resistance, and altered metabolic response. The sicker the patient, the higher the threat to nutrition and micronutrient status. In addition, many patients start the ICU stay with an altered nutrition status, which requires assessment upon admission. Nutrition needs vary among patients as well as during hospitalization, as the metabolic response changes over time. Shock and acute organ failure result in a metabolic shift toward intense catabolism: endogenous glucose production aiming at ensuring the basal adenosine triphosphate production starts immediately and occurs at the expense of the lean body mass using amino acids for neoglucogenesis. Later, the stabilization and recovery phases are characterized by higher energy and substrate needs. Indirect calorimetry is the only tool enabling determination of the metabolic level. When and how should feeding be started? Recent research shows that the route does not matter much, with equipoise between enteral and parenteral nutrition (PN) as long as overfeeding is avoided. As micronutrients are an integral part of metabolism and antioxidant defenses, their delivery must be ensured: whereas needs are well defined for healthy individuals, needs for illness remain poorly defined. PN that contains only macrosubstrates requires the daily prescription of multimicronutrient complements to qualify as total PN. Achievement of goals requires minimal monitoring, consisting of the daily verification of energy and protein goal delivery achievement and daily follow-up determining blood glucose and phosphate levels and insulin requirements.