The studies carried out during the last two decades have represented a great effort in trying to identify and define cell populations endowed with the phenotypic and functional properties of endothelial progenitors. From these studies a scenario now emerges indicating that in the blood there are very rare endothelial progenitor cells, called endothelial colony-forming cells (ECFCs) or late outgrowth endothelial cells, not originated from bone marrow, capable of generating phenotypically and functionally competent endothelial cells, capable to be incorporated in vivo into growing vessels. ECFCs are present in the circulation as well as cells resident in the vascular endothelial intima. In addition to these progenitors, there are some hematopoietic progenitor cells capable of generating a monocytic cell progeny exerting a pro-angiogenic activity in vivo, but unable to be directly incorporated into growing vessels. These cells exert a pro-angiogenic effect in vivo through a paracrine mechanism based on the secretion of growth factors and cytokines.