Acute upper gastrointestinal bleeding afflicts thousands of patients each year, and can range from a mere nuisance to a life threatening condition. The underlying cause is most often peptic ulcer disease, gastritis, or varices. Infrequent causes include esophagitis, cancer, aorto-enteric fistulae, hemobilia, and arteriovenous malformations. This chapter elaborates on the rapid assessment and risk stratification of patients presenting with acute upper gastrointestinal bleeding. We discuss how endoscopy and other modalities aid in the localization and treatment of the bleeding source.