Hereditary haemorrhagic telangiectasia (HHT) affects one in 5-8000, and no longer can be viewed as solely causing anaemia (due to nasal and gastrointestinal bleeding) and characteristic mucocutaneous telangiectasia. Arteriovenous malformations commonly occur, and in the pulmonary and cerebral circulations demand knowledge of risks and benefits of asymptomatic screening and treatment. HHT is inherited as an autosomal dominant trait and there is no age cut off when apparently unaffected offspring of an individual with HHT can be told they are unaffected. This review focuses on the evolving evidence base for HHT management, issues regarding pregnancy and prothrombotic treatments, and discusses the molecular and cellular changes that underlie this disease.