Regeneration and fibrosis are integral parts of the recovery from tissue injury, and impaired regulation of these mechanisms is a hallmark of many chronic diseases. Traditionally, resident tissue fibroblasts have been thought to proliferate and mediate local fibrosis. However, more recently, data suggest that a circulating bone-marrow-derived progenitor cell, the fibrocyte, plays a critical role in the repair of injured tissue in a diverse set of disease states, including fibrotic lung disease. In this chapter, we describe the unique characteristics of fibrocytes, how they are recruited to the site of lung injury, and experimental results supporting their pivotal role in lung repair and fibrosis.