During embryogenesis, endothelial progenitor cells participate in the initial processes of primitive blood vessel formation (vasculogenesis). It has become evident that progenitors to vascular endothelial cells also exist in the adult. Endothelial progenitors normally reside in the adult bone marrow but may become mobilized into circulation by cytokine or angiogenic growth factor signals from the periphery, enter extravascular tissue, and promote de novo vessel formation by virtue of physically integrating into vessels and/or supplying growth factors (adult vasculogenesis). For that reason, autologous endothelial progenitors, mobilized in situ or transplanted, has become a major target of therapeutic revascularization approaches to ischemic disease and endothelial injury. Moreover, endothelial progenitors represent a potential target of strategies to block tumor growth.