Appropriate nutritional support has long been considered essential for the recovery of postoperative, critically ill and injured human patients. Whilst there is convincing evidence of the deleterious effects of malnutrition in people, the optimal nutritional strategies for critically ill and postoperative animals remain controversial and are largely unknown. Despite the lack of definitive answers, it must be emphasized that recommendations for nutritional support of critically ill animals are based on current understanding of the metabolic response to injury and the limited clinical information available. This should not discourage the implementation of nutritional support for critically ill or injured animals. In fact, with proper patient selection, sound nutritional planning and careful monitoring, nutritional support can be an integral part of the successful recovery of many critically ill animals. This chapter covers Metabolic responses; Identifying the need for nutritional support; Nutritional assessment; Nutritional plan; Modes of nutritional support; and Complications of providing nutrition in the critically ill.