Occult gastrointestinal bleeding, defined as bleeding that is unknown to the patient, is the most common form of gastrointestinal bleeding and can be caused by virtually any lesion in the gastrointestinal tract. Patients with occult gastrointestinal bleeding include those with fecal occult blood and iron-deficiency anemia (IDA). In men and postmenopausal women, IDA should be considered to be the result of gastrointestinal bleeding until proven otherwise. Indeed, the possibility of gastrointestinal tract malignancy in these patients means that gastrointestinal evaluation is nearly always indicated. Obscure gastrointestinal bleeding is defined as obvious bleeding from a difficult to identify source and is always recurrent. This form of bleeding accounts for approximately 5% of all cases of clinically evident gastrointestinal bleeding and is most commonly caused by bleeding from the small intestine. Capsule endoscopy and deep enteroscopy have had a major impact on the way that patients with occult and, in particular, obscure bleeding are managed. In this Review the causes, diagnostic evaluation and treatment of occult and obscure gastrointestinal bleeding are discussed.