Critically ill patients are at high risk of malnutrition, due to the nature of their illness and hypermetabolic catabolic state. This chapter presents an assessment of the nutritional status and nutritional requirements by calculating resting energy expenditure for caloric requirements, calculating protein requirements, calculating non-protein (carbohydrates and lipids) components, and by calculating micronutrients including vitamins, electrolytes and trace elements. Nutritional support can be given through one of two routes: enteral feeding (EF) (via the gastrointestinal tract) or parenteral feeding (PN), intravenous (via either peripheral or central vein). Pharmaco/immunonutrition is a relatively new concept in critical care feeding. Some ICUs now have protocols for the use of immunonutrition feeds. The chapter discusses the different complications of nutritional support such as refeeding syndrome, overfeeding, hyperglycaemia, electrolyte imbalances and micronutrient deficiency, and different complications of enteral nutrition, and parenteral nutrition.