Respiratory failure and the need for mechanical ventilation brought about by a variety of medical, surgical and traumatic events makes the optimum nutritional requirements of this group of patients difficult to determine. Nonetheless, support is an important adjunct to the management of patients in the intensive care unit, and mechanically ventilated patients being especially vulnerable to complications of under- or over-feeding. This chapter considers the nutritional requirements, route and timing of nutritional support, and complications associated with feeding mechanically ventilated, critically ill patients. Compared with conventional insulin therapy, intensive insulin therapy was associated with improvements in renal function, duration of mechanical ventilation and discharge from intensive care unit (ICU) and from the hospital. Despite the lack of definitive trials demonstrating clinically meaningful benefit from nutritional support, there is strong evidence that malnutrition is associated with a worse outcome. In addition, under-feeding and over-feeding have had undesirable consequences.