Nutrition plays an important role in the pathogenesis, treatment, and morbidity of Crohn disease. Approximately two thirds to three fourths of hospitalized patients with active disease and one fourth of outpatients with Crohn disease are malnourished. Malnutrition, which can be present even when Crohn disease is in remission, can affect growth, cellular and humoral immunity, bone density, and wound healing. Decreased nutrient intake, malabsorption, drug-nutrient interactions, anorexia, and protein-losing enteropathy can all contribute to the protein-calorie malnutrition and other specific nutrient deficiencies seen in Crohn disease. Therefore, by preventing and correcting nutrient deficiencies, nutritional therapy is an important component in the overall management of patients with Crohn disease.