Acute mesenteric ischemia is uncommon and always occurs in the setting of preexisting comorbidities. Mortality rates remain high. The 4 major types of acute mesenteric ischemia are acute superior mesenteric artery thromboembolic occlusion, mesenteric arterial thrombosis, mesenteric venous thrombosis, and nonocclusive mesenteric ischemia, including ischemic colitis. Delays in diagnosis are common and associated with high rates of morbidity and mortality. Prompt diagnosis requires attention to history and physical examination, a high index of suspicion, and early contract CT scanning. Selective use of nonoperative therapy has an important role in nonocclusive mesenteric ischemia of the small bowel and colon.