The autosomal-dominant trait hereditary haemorrhagic telangiectasia (HHT) affects 1 in 5–8000 people. Genes mutated in HHT (most commonly for endoglin or activin receptor-like kinase (ALK1)) encode proteins that modulate transforming growth factor (TGF)-β superfamily signalling in vascular endothelial cells; mutations lead to the development of fragile telangiectatic vessels and arteriovenous malformations. In this article, we review the underlying molecular, cellular and circulatory pathobiology; explore HHT clinical and genetic diagnostic strategies; present detailed considerations regarding screening for asymptomatic visceral involvement; and provide overviews of management strategies.