The small intestine is a convoluted tube extending from the pylorus to the ileocecal valve. It is about 7 m long, and gradually diminishes in size from its commencement to its termination. The small intestine is divisible into three portions: the duodenum, the jejunum, and the ileum. In this chapter we will focus on the jejunum and the ileum. The upper 40 % constitutes the jejunum and the remainder is the ileum. There is no morphological line of distinction between the two and the division is arbitrary. However there are subtle changes that make this distinction possible.The jejunum has a thicker wall and a wider lumen whereas the ileum has a thinner wall and a smaller lumen. Adipose tissue is more abundant in the mesentery of the ileum, thus vessels in the mesentery are not as well visualized. The small intestinal mesentery is fan-shaped with a root of about 15 cm extending obliquely from the left L2 lumbar vertebral transverse process level to the right sacroiliac joint and crossing over the third portion of the duodenum, the aorta, inferior vena cava, and the right ureter. Between the two leaves of the mesentery are the mesenteric vessels and lymph nodes. The superior mesenteric artery (SMA) is the artery of the small intestine. The jejunum has fewer (2-3) series of vascular arcades per segment and the vasa recta are longer; whereas, the ileum has more (4-5) series of arcades and the vasa recta are shorter.The proximal jejunum and distal ileum are more important for absorption than the rest of the small intestine. Massive resection of small bowel (e.g., in mesenteric vascular disease) or repeated resections (e.g., in Crohn's disease) may result in short bowel syndrome and malabsorption. Resection of a signifi cant portion of the terminal ileum is associated with a loss of absorption of the fat soluble vitamins (i.e., vitamins A, D, E, K, and B12). Major ileal resection also creates changes in cholesterol metabolism and bile acid reabsorption. If undigested fats and bile acids transit directly into the transverse colon, the osmotic pressure of the colon is elevated and diarrhea may result.