Circulating blood contains a subtype of progenitor cells that have the capacity to differentiate into mature endothelial cells in vitro and in vivo. These cells have been termed endothelial progenitor cells (EPCs). The isolation of EPCs by adherence culture or magnetic microbeads has been described. EPCs are characterized by the expression of 3 markers, CD133, CD34, and the vascular endothelial growth factor receptor-2. After differentiation, EPCs express CD31, vascular endothelial cadherin, and von Willebrand factor. Evidence is accumulating that EPCs can facilitate endothelial repair and angiogenesis in vivo. We observed that EPCs can regenerate damaged endothelial cells in vascular grafts in apoE-deficient mice, and that abundant vascular progenitor cells are present in the adventitia of the vessel wall. It is not clear yet, however, whether these EPCs are essential for these angiogenic and atherogenic processes. Moreover, there are still many uncertainties about how cardiovascular risk factors alter EPC function. Thus, further studies on the mechanisms of EPC homing, releasing and attaching will be of help to explore areas of potential basic research and clinical application of EPCs.