Fully differentiated endothelial cells and their precursors circulate in the bloodstream. Since their initial description more than 30 years ago, circulating endothelial cells have been quantified in a number of different clinical conditions that affect the endothelium. Only recently, however, have investigators begun to examine the protein expression and functionality of these cells. Because a number of diseases prevalent in the field of nephrology affect endothelial cells, the study of circulating endothelial cells may allow the direct examination of the state of the endothelium in these conditions. This review will discuss the endothelium and renal disease, the methods to quantify these circulating endothelial cells, their origins, and their therapeutic potential.