In this radiology lecture, we discuss the appearance of gallstone ileus on x-ray and CT. Key points include: * Gallstone ileus is a rare complication of chronic cholecystitis. * Actually not an ileus, but a small bowel obstruction. * Gallstone migrates through a fistula between gallbladder and small bowel (usually duodenum) and becomes impacted in the terminal ileum. * Stone can also impact in the proximal ileum, jejunum, even in the duodenum/distal stomach causing gastric outlet obstruction (Bouveret syndrome). * Rigler triad on abdominal x-ray: Small bowel obstruction, pneumobilia and gallstone in the right iliac fossa. * Usually affects the elderly and treated surgically.