Endoglin (CD105) is an accessory receptor for transforming growth factor beta (TGF-β) and its expression is up-regulated in actively proliferating endothelial cells. Endoglin has been suggested as an appropriate marker for tumor-related angiogenesis and neovascularization. Several studies demonstrate the potential of endoglin in tumor diagnosis, prognosis, and therapy. This review details the structure and function of endoglin, and investigates the role of endoglin in angiogenesis and tumor diagnosis, prognosis, and therapy.