Acute mesenteric ischemia is interruption of intestinal blood flow by embolism, thrombosis, or a low-flow state. Bowel infarction is the end result of a process initiated by mediator release and inflammation. On clinical assessment, the early hallmark is severe abdominal pain but minimal physical findings. The abdomen remains soft, with little or no tenderness. Mild tachycardia may be present. Early diagnosis is difficult, but selective mesenteric angiography and computed tomographic angiography have the most sensitivity; other imaging studies and serum markers can show abnormalities but lack sensitivity and specificity early in the course of the disease, when diagnosis is most critical. Treatment is by embolectomy, anticoagulation, revascularization of viable segments, or resection; sometimes vasodilator therapy is successful. If diagnosis and treatment take place before infarction occurs, mortality is low; after intestinal infarction, mortality approaches 30 to 70%. For this reason, in the emergency department, clinical diagnosis should supersede diagnostic tests, which may delay treatment. This review contains 6 highly rendered figures, 4 tables, and 33 references. Key words: acute mesenteric ischemia; bowel necrosis; chronic mesenteric ischemia; mesenteric occlusive disease; mesenteric venous thrombosis; nonocclusive mesenteric ischemia; postprandial abdominal pain; superior mesenteric artery thromboembolism