The roles of circulating endothelial cells (CECs) and circulating endothelial progenitors (CEPs) are currently being investigated in several diseases including cancer and metastases development. Preclinical and clinical data suggest that CEC enumeration might be useful to identify patients who might benefit from anti-angiogenic treatments while CEPs seem to have a "catalytic" role in different steps of cancer progression and recurrence after therapy. The definition of CEC and CEP phenotypes and the standardization of CEC and CEP enumeration procedures are highly warranted to use these cells as biomarkers in clinical trials in oncology, and to compare results from different studies.