This chapter discusses the diagnosis, evaluation and management of gastrointestinal bleeding including upper gastrointestinal bleeding (UGIB) and lower gastrointestinal bleeding (LGIB). It describes the special circumstances with regard to aortoenteric fistula, liver disease, and Jehovah's Witnesses. In UGIB, patients typically present with hematemesis, coffee-ground emesis, and/or melena. In LGIB, patients typically present with bright red blood per rectum (BRBPR), also known as hematochezia. Higher severity of disease is indicated by signs of shock such as hypotension, tachycardia, altered mental status (AMS), decreased urine output (UOP), cool skin, syncope, orthostasis. Change in pulse with posture is more sensitive than hypotension, but may be masked by medications (e.g., beta-blockers). Hypotension, tachycardia, and tachypnea can indicate hemorrhagic shock and requires immediate treatment. Massive transfusion protocols are helpful for significant bleeding in order to prevent further coagulopathy due to transfusion of high volume of crystalloid or only red cells.