Lower gastrointestinal (GI) hemorrhage is a common clinical condition often encountered by the acute care surgeon. Lower GI bleeding, defined as bleeding distal to the ligament of Treitz, may present with diverse manifestations, from occult bleeding as evidenced only by anemia to massive hemorrhage and exsanguination. Severe, life-threatening hemorrhage may present precipitously with few initial symptoms. As such, the astute surgeon must be able to expeditiously identify patients with acute, massive lower GI bleeding and initiate the appropriate therapeutic algorithm to reduce the high morbidity and mortality associated with this condition. After initial resuscitation, the cause of the hemorrhage must be identified. Identification of the bleeding site often includes a multidisciplinary approach, including practitioners from critical care, gastroenterology, radiology, and surgery. In general, the primary methods to locate the site of hemorrhage include CT and endoscopy. Advances in endoscopic localization have increased both the diagnostic and therapeutic yields of such therapy. Surgical intervention is generally reserved for those patients in whom hemodynamic instability precludes further diagnostic workup or those in whom the source of bleeding cannot be controlled with other modalities. In this review, we discuss the diagnostic workup and therapeutic management of life-threatening lower GI hemorrhage. This review contains 10 figures, 3 tables and 93 references Key words: BLEED criteria, colonic ischemia, colonoscopy, CT angiography, diverticular disease, lower gastrointestinal bleeding, mesenteric arteriography, nuclear scintigraphy, push enteroscopy, video capsule endoscopy