Patients in the intensive care unit (ICU) are unable to nourish themselves orally. In addition, critical illness increases nutrient requirements as well as alters metabolism. Typically, ICU patients rapidly become malnourished unless they are provided with involuntary feeding either through a tube inserted into the GI tract, called enteral nutrition (EN), or directly into the bloodstream, called parenteral nutrition (PN). Between the 1960s and the 1980s, PN was the modality of choice and the premise was that if some is good, more is better, which led to overfeeding regimens called hyperalimentation. Later, the dangers of overfeeding, hyperglycemia, fatty liver, and increased sepsis associated with PN became recognized. In contrast, EN was not associated with these risks and it gradually became the modality of choice in the ICU. However, ICU patients in whom the gastrointestinal tract was nonfunctional (i.e., gut failure) required PN to avoid malnutrition. In addition, EN was shown, on average, to not meet nutrient requirements, and underfeeding was recognized to increase complications because of malnutrition. Hence, the balanced perspective has been reached of using EN when possible but avoiding underfeeding by supplementing with PN when required. This new role for PN is currently being debated and studied. In addition, the relative merits and needs for protein, carbohydrates, lipids, and micronutrients are areas of study.