The small intestine comprises the duodenum, jejunum and ileum. On a lateral abdominal radiograph, the small intestine occupies most of the mid-ventral abdomen, lying caudal to the stomach and cranial to the bladder. It appears as smoothly curving ‘tubes’ in long axis views and as circular or ring-shaped opacities in cross section. Peristaltic waves cause transient segmental narrowing, which may be recognized on plain radiographs. The duodenum runs cranially and laterally from the pylorus and then turns caudally, forming the cranial duodenal flexure. The flexure is held against the caudal surface of the right liver lobes by the hepatoduodenal ligament. The descending duodenum runs dorsally and caudally along the right abdominal wall, before turning medially at the caudal duodenal flexure. The ascending duodenum runs cranially and to the left of midline, where it becomes the jejunum at the duodenojejunal junction. The duodenum may occasionally be recognized on a plain radiograph from its characteristic position; it is relatively fixed compared with the jejunum and ileum, which have a long mesentery and are readily displaced by changes in adjacent structures. The duodenum is also slightly wider than the jejunum and ileum. The terminal ileum may be recognized in some cases at the ileocaecocolic junction, which lies at approximately the level of the fourth lumbar vertebra on a lateral abdominal view. On a ventrodorsal (VD) view, the caecum lies on the right at the level of the second to fourth lumbar vertebrae. The jejunum and ileum are otherwise radiographically indistinguishable. Normal radiographic anatomy; Contrast radiography; Ultrasonography; Overview of additional imaging modalities; and Intestinal diseases are discussed in this section.