Malnutrition frequently contributes to the immunocompromise seen in hospitalized patients. Nutritional support corrects malnutrition and can reverse the associated immunocompromise. Developing an understanding of nutritional needs and the role of nutrition in immune function is essential to prevention and treatment of nutrition-related immunocompromise. Current research is defining the role of specific nutrients in immune function. Recent evidence also suggests that the route (enteral versus parenteral) of providing rutritional support can affect immune competence. Intervention trials may show a role of key nutrients in not only maintaining normal immune competence, but also in modulating immunologic outcomes in critically ill patients.