Emerging evidence indicates that bone marrow (BM)-derived endothelial progenitor cells (EPCs) contribute to angiogenesis-mediated growth of certain tumors in mice and human. EPCs regulate the angiogenic switch via paracrine secretion of proangiogenic growth factors and by direct luminal incorporation into sprouting nascent vessels. While the contributions of EPCs to neovessel formation in spontaneous and transplanted tumors and to the metastatic transition have been reported to be relatively low, remarkably, specific EPC ablation in vivo has resulted in severe angiogenesis inhibition and impaired primary and metastatic tumor growth. The existence of a BM reservoir of EPCs, and the selective involvement of EPCs in neovascularization, have attracted considerable interest because these cells represent novel target for therapeutic intervention. In addition, EPCs are also being used as pharmacodynamic surrogate markers for monitoring cancer progression, as well as for optimizing efficacy of anti-angiogenic therapies in the clinic. This review will focus primarily on recent advances and emerging concepts in the field of EPC biology and discuss ongoing debates involving the role of EPCs in tumor neovascularization. For detailed information on the in vitro characterization of EPCs contribution to non-tumor pathologies, the reader is directed towards several excellent reviews and publications [F. Bertolini, Y. Shaked, P. Mancuso and R.S. Kerbel, Nat. Rev., Cancer 6 (2006) 835-845. [1]] [J.M. Hill, T. Finkel and A.A. Quyyumi, Vox Sang. 87 Suppl 2 (2004) 31-37. [2]] [A.Y. Khakoo and T. Finkel, Annu. Rev. Med. 56 (2005) 79-101. [3]] [H.G. Kopp, C.A. Ramos and S. Rafii, Curr. Opin. Hematol. 13 (2006) 175-181. [4]; K.K. Hirschi, D.A. Ingram and M.C. Yoder, Arterioscler. Thromb. Vasc. Biol. 28 (2008) 1584-1595. [5]; F. Timmermans, J. Plum, M.C. Yoder, D.A. Ingram, B. Vandekerckhove and J. Case, J. Cell. Mol. Med. 13 (2009) 87-102. [6]] and reviews by Bertolini, Voest and Yoder in this issue.