Acute upper gastrointestinal bleeding (UGIB) is a common and potentially life-threatening emergency. Despite significant advances in intensive care resuscitation, medical treatment of gastric acid hypersecretion, and progress in endoscopic and surgical management, mortality from upper gastrointestinal hemorrhage has remained steady over the past four decades. One of the major challenges of managing UGIB involves identifying patients who are at high risk for rebleeding and death and who require admission to the intensive care unit. Regardless of the cause, initial evaluation of patients with UGIB is based on the degree of hemodynamic instability and the presumed rate of bleeding. Those patients with evidence of active bleeding and hemodynamic instability require aggressive resuscitation and hospitalization. Although diagnostic imaging may be useful in identifying the source of bleeding, endoscopy remains the “gold standard” diagnostic and therapeutic modality. Recent advances in transcatheter angiographic embolization have made this modality an attractive alternative to surgical intervention in patients who fail endoscopic management. However, in the hemodynamically unstable patient, surgical intervention is often necessary. In this review, we describe the most common causes of acute UGIB and detail the initial workup and management of each cause. This review contains 6 figures, 3 tables, and 71 references. Key words: acute upper gastrointestinal bleeding, angiographic embolization, Billroth, Dieulafoy, esophagogastroduodenoscopy, peptic ulcer disease, scintigraphy, varices