Nutritional support early during the postoperative period or after onset of illness decreases the mortality rate and shortens the duration of hospitalization of dogs and cats. The preferred feeding route is dependent on the condition of the patient. If there are no contraindications, every patient must receive nutritional support, at least consistent with the resting energy requirement (RER). Several nutrients may be beneficial during healing and recovery from illness or surgery, but further research is needed to empirically certify the effects of these nutrients in critically ill patients.