Early endoscopy has been advocated for the management of upper gastrointestinal bleeding, but the optimal timing for early endoscopy is still uncertain. The aim of this Review is to evaluate the optimal timing of early endoscopy by examining the findings of randomized clinical trials and retrospective cohort studies that used comparable outcome measures and have been reported in the literature. Outcome measurements included recurrent bleeding, surgery, mortality, length of hospital stay, and blood transfusion. Studies were categorized into those in which endoscopy was performed within 2-3 h, 6-8 h, 12 h or 24 h of the patient's presentation to hospital. We conclude that early endoscopy aids risk stratification of patients and reduces the need for hospitalization. However, it may also expose additional cases of active bleeding and hence increase the use of therapeutic endoscopy. No evidence exists that very early endoscopy (within a few hours of presentation) can reduce the risk of rebleeding or improve survival.