Injury followed by repair is common to many lung diseases. In some conditions such as the adult respiratory distress syndrome, lung injury is acute and massive, and both the injury and the exuberant repair response which follows can compromise lung function. In pulmonary emphysema, chronic low-grade tissue injury exceeds the capacity of the lung for repair, and net tissue destruction results. In chronic bronchitis, current concepts suggest that injury of the airways followed by attempts at repair leads to a number of consequences. These include metaplasia of the airway epithelium with alterations in mucus production and mucociliary clearance as well as development of fibrosis in the airway wall. Contraction of these fibrotic scars results in airway narrowing which can compromise airflow. COPD, therefore, is characterized both by inadequate repair in emphysema and excessive or disordered repair in bronchitis. This chapter will review current understanding of repair processes in the lung, particularly as they apply to COPD.