Since the discovery of the circulating fibrocyte as a collagen-producing cell of the peripheral blood, the physiologic and pathologic role of this unique cell population has grown steadily. The present review summarizes the known biology of fibrocytes and discusses evidence for their role in the pathogenesis of nephrogenic systemic fibrosis. Possible mechanisms by which gadolinium may influence the activation or trafficking properties of fibrocytes leading to tissue fibrosis are discussed.