Nutrition plays a role in inflammatory bowel disease (IBD) primarily in prevention and treatment of malnutrition and growth failure. Furthermore, in Crohn disease (CD), nutrition can induce remission, maintain remission, and prevent relapse. Malnutrition is common in IBD and the mechanisms involved include decreased food intake, malabsorption, increased nutrient loss, increased energy requirements, and drug-nutrient interactions. At the time of diagnosis, up to 85% of pediatric patients with CD and 65% of those with ulcerative colitis (UC) have weight loss. Growth failure occurs in 15% to 40% of children with IBD and is less common in UC compared with CD, both at diagnosis and during follow-up. In CD, nutritional therapy with enteral formulas induces remission at a rate comparable with that achieved with steroids. In adults with CD, limited information suggests that enteral nutrition (EN) may play a role in maintenance of remission. In children with CD colitis, one study suggested that children without colitis respond better to EN than children with colitis, and another study found no such difference but reported a trend toward earlier relapse in those with isolated colonic involvement. Finally, nutrition may play a role in IBD via the possible protective effect of breastfeeding against UC and CD. In summary, although only CD may benefit from nutrition as primary therapy for remission induction and possibly maintenance of remission, nutrition plays an important role in the prevention and treatment of malnutrition in IBD, and may have a protective role, via the effect of breast-feeding on disease occurrence.