Tissue regeneration and repair are fundamental both to recovery of the lung from injury and to the pathology of many chronic lung diseases. There are two potential sources for the adult progenitor cells that participate in this reparative process: resident lung progenitors and bone marrow-derived circulating cells. Bone marrow-derived cells, in particular, have been shown to give rise to airway and alveolar epithelial cells, as well as lung mesenchymal cells. Emerging data have linked specific chemokine ligand-receptor interactions to the recruitment of these cells to the lung and has implicated these cells in chronic lung disorders such as asthma and interstitial lung diseases. In this review, we summarize the current understanding of the biology of adult circulating progenitors as related to lung disease.