An increased number of circulating endothelial cells (CECs) and endothelial progenitor cells (CEPs) has been reported in cancer patients. CEPs are derived from the bone marrow and will, during angiogenesis, differentiate into endothelial cells. CECs are mature endothelial cells (ECs) released from the vessel intima during physiological endothelial turnover or as a result of tumor treatment. Preclinical studies have shown that during tumor progression, the amount of circulating CECs correlates with angiogenesis. Moreover, there is growing evidence suggesting that CECs and CEPs viability and kinetics correlate with the patient responses to anti-angiogenic therapies. Thus, circulating CECs and CEPs may act as surrogate markers to test putative therapeutic efficacy. Moreover measuring CECs and CEPs may be useful to assess effects of antiangiogenic therapy.