The remodelling of existing vessels (i.e. angiogenesis) and the "de novo" vessel formation (i.e. vasculogenesis) occur not only during the embryonic development but also over the entire postnatal life. In 1997, the Asahara group first reported that endothelial progenitor cells circulate in peripheral blood and that they are recruited at sites of ischemia, thus proving that these cells are able to promote vasculogenesis. Since then, several different approaches have been set up to investigate endothelial progenitor cells. This review summarizes the different modalities utilized to enumerate these cells, delineates their involvement in the haemostatic pathways, and depicts their altered trafficking during cardiovascular diseases. Finally, recent observations suggesting a primary role for endothelial progenitors in particular situations such as pulmonary arterial hypertension or Philadelphia negative-myeloproliferative neoplasms are discussed.