Morbus Osler or hereditary haemorrhagic telangiectasia (HHT) is a genetic disorder resulting in development of arteriovenous malformations in the mucosa and in visceral organs. The most common symptom is epistaxis. The disease may, however, cause a variety of other serious manifestations such as pulmonary arteriovenous malformations (PAVM), cerebral arteriovenous malformations (CAVM) and gastrointestinal bleeding. Collaboration between various medical specialties is essential in order to provide an up-to-date treatment and thorough work-up in the individual patient. Disregard of symptoms may result in substantial morbidity and may have serious consequences. This overview presents our current understanding of HHT.