Critically ill patients are at high risk of malnutrition and lean body mass loss. Screening for malnutrition and performing detailed assessment of energy needs should be routine for patients admitted to intensive care units. Providing adequate calorie and protein provisions can attenuate muscle loss in many at-risk patients. Enteral nutrition is associated with decreased risks of morbidity and infections and is therefore preferred to parenteral nutrition in hemodynamically stable patients with favorable anatomy. Judicious use of steroids and paralytics in combination with adequate glucose control may decrease the risk of developing critical illness polyneuromyopathy. There is growing evidence for the potential immune-enhancing benefits of many micronutrients and vitamins in the critically ill, but more research is needed to determine which nutrients are most effective in which disease processes and what dosing regimens are safe and effective. Elderly, obese, and very young patients pose unique challenges for nutrition therapy and early mobility programs. Pairing early mobility programs with optimal nutrition therapy can help reduce morbidity, limit muscle loss, and speed recovery in intensive care unit patients.