Chronic obstructive pulmonary disease (COPD) is defined in this chapter, and the relation between its two major components, (a) chronic bronchitis and emphysema and (b) nonremitting asthma, is discussed. Intensity of tobacco smoking and age are the major risk factors for the development of chronic airways obstruction. Environmental air pollution, childhood infections, and familial factors other than alpha-1 protease inhibitor deficiency appear to play only small roles. Emphysema is the major cause of severe airways obstruction; bronchiolitis is a contributing factor and likely is responsible for the minor reversible element of airways obstruction. The elastase-antielastase hypothesis, which is based mainly on indirect evidence, is the best explanation for the pathogenesis of emphysema. Extensive airspace enlargement with fibrosis is infrequently observed; this mechanism may play a role in the pathogenesis of the centrilobular emphysema of smokers.