In the embryo, the mesodermal precursor cell, the hemangioblast, gives rise to blood and blood vessels. During adult life, the hematopoietic stem cell (HSC) also exhibits this bipotential hemangioblast activity, serving as a rich source for circulating endothelial progenitor cells (EPCs). As a result of this finding, many questions have arisen as to whether the adult HSC is involved in the day-to-day maintenance of tissues, what mechanisms influence this adult hemangioblast activity, and whether blood vessels harbor hematopoietic capability. In answering these questions, the power of adult hemangioblast activity could be harnessed to evaluate and treat diseases such as myocardial infarction, stroke, cancer, and blindness. Enumeration of activated EPCs aims to alert the patient as to the severity of their disease, predict response to therapy, and gauge for relapse potential. Identification of hemangioblast stimulatory or inhibitory cues would allow physicians to regulate neovascularization in their patient, augmenting vessel production in situations of hypo-proliferation such as wound healing and inhibiting vessel production in situations of hyper-proliferation such as cancer. Finally, given that EPCs home to sites of new blood vessel growth, genetic engineering of harvested HSC or EPC offers the potential to deliver vasoregulatory factors directly to sites of neovascularization.