The ileum is the third section of the small intestine after the duodenum and the jejunum; it is 2 to 4 meters long. The ileum is where digestion is completed by enzymes that break down carbohydrates, fats, and proteins. Its inner wall is lined with numerous finger-like projections (villi), which increase the surface area available for the absorption of nutrients. The ileum is separated from the large intestine by the ileocaecal valve.

Disorders commonly associated with the ileum include irritable bowel syndrome and Crohn's disease.