Endothelial progenitor cells (EPCs), present in the blood and bone marrow, represent a potential source of endothelial cells for repair of injured blood vessels, neovascularization, and tissue engineering. EPCs are present at low levels in peripheral blood, although their numbers increase in response to cytokines, VEGF, and statins. There are at least two types of EPCs characterized following in vitro culture: colony-forming unit ECs (CFU-ECs) and endothelial colony-forming cells (ECFCs). CFU-ECs appear early in culture, have limited ability to proliferate, and share markers for endothelial cells and monocytes. In contrast, ECFCs appear later in culture, grow rapidly, and to large numbers express only endothelial cell markers. This chapter examines the properties of these EPCs, in vitro and in vivo studies using these two cell types, and the potential of these EPCs for therapeutic applications. © 2011 Springer-Verlag Berlin Heidelberg.