Upper gastrointestinal (UGI) bleeding refers to bleeding arising from the gastrointestinal tract proximal to the ligament of Treitz, mainly from the oesophagus, stomach, and duodenum. It is a potentially life-threatening critical-care condition that may present with haematemesis, coffee-ground vomiting, or melena. The causes can be broadly classified into oesophageal, gastric, duodenal, vascular, drug-related, and systemic causes. Peptic ulcer disease is one of the most common causes of upper gastrointestinal bleeding. Gastric and duodenal ulcers can erode blood vessels in the gastrointestinal wall and produce significant haemorrhage. The major risk factors are *Helicobacter pylori* infection, prolonged use of non-steroidal anti-inflammatory drugs (NSAIDs), corticosteroid use in combination with other risk factors, smoking, and severe physiological stress. In critically ill patients, stress-related mucosal disease may cause acute gastric or duodenal erosions and bleeding. Oesophageal varices are another important cause, particularly in patients with chronic liver disease and portal hypertension. Increased pressure in the portal venous system causes dilatation of submucosal veins in the lower oesophagus. These fragile vessels may rupture suddenly and result in massive haematemesis. Gastric varices may similarly rupture and cause severe bleeding. Such bleeding is an emergency because rapid blood loss can lead