Endoglin is a transmembrane auxillary receptor for transforming growth factor-beta (TGF-beta) that is predominantly expressed on proliferating endothelial cells. Endoglin deficient mice die during midgestation due to cardiovascular defects. Mutations in endoglin and activin receptor-like kinase 1 (ALK1), an endothelial specific TGF-beta type I receptor, have been linked to hereditary hemorrhagic telangiectasia (HHT), an autosomal dominant vascular dysplasia characterized by telangiectases and arteriovenous malformations. Endoglin heterozygote mice develop HHT-like vascular abnormalities, have impaired tumor and post-ischemic angiogenesis and demonstrate an endothelial nitric oxide synthase-dependent deterioration in the regulation of vascular tone. In pre-eclampsia, placenta-derived endoglin has been shown to be strongly upregulated and high levels of soluble endoglin are released into the circulation. Soluble endoglin was found to cooperate with a soluble form of vascular endothelial growth factor receptor 1 in the pathogenesis of pre-eclampsia by inducing endothelial cell dysfunction. Endoglin is highly expressed in tumor-associated endothelium, and endoglin antibodies have been successfully used to target activated endothelial cells and elicit anti-angiogenic effects in tumor mouse models. These exciting advances provide opportunities for the development of new therapies for diseases with vascular abnormalities.