Cigarette smoking clearly has been shown to be the major environmental risk factor predisposing to the development of COPD. Occupational exposures to dust and fumes, air pollution, passive smoke exposure, childhood respiratory infections, and diet may also contribute. Airway hyperresponsiveness is a risk factor for the development of decline in FEV1, but its role in the development of COPD remains uncertain. Alpha1-antitrypsin deficiency is an important genetic risk factor for COPD in the small minority of COPD patients who inherit this deficiency. Other genetic factors are likely involved but have not yet been identified. Elucidation of additional genetic risk factors may provide useful insights into the pathogenesis of COPD. Potential interactions between the various environmental and genetic risk factors may be extremely important in determining the variable development of COPD.