Hereditary hemorrhagic telangiectasia (HHT), also known as Osler-Weber-Rendu disease, is a hereditary disorder leading to easily bleeding telangiectases on skin and mucosal surfaces, and it is associated with the presence of arteriovenous malformations (AVMs) in multiple organ systems. These AVMs may cause serious complications when they are located in the lungs, liver, or brain. The prevalence of AVMs in patients with HHT might be higher than previously estimated. Nowadays, treatment is often possible. In some families, mutations have been shown in the gene encoding for a transforming growth factor receptor, endoglin. Genetic heterogeneity has been demonstrated, suggesting involvement of other transforming growth factor receptors. This might explain the variable clinical expression of the disease. In view of the high prevalence of pulmonary and cerebral AVMs, all patients with HHT should be screened for their presence, and relatives of patients with HHT should be investigated for presence of the disease.