Circulating endothelial cells (CEC) have evolved as a novel method of assessing endothelial function. Rarely found in the peripheral blood of healthy individuals, CECs increase in a wide variety of conditions such as cardiovascular disease, cancer, infection and inflammatory states. Unlike immature endothelial progenitor cells (EPC), which are bone marrow derived, CECs are felt to represent the detachment of mature endothelial cells from the endothelial monolayer, as a result of endothelial insult. In this article we present an overview of CECs and their association with a broad spectrum of disease processes.