Abstract An outline of previous work on the pathological anatomy of emphysema and the natural history of pulmonary inflammation is presented, with the aim of producing a satisfying account of the pathogenesis of emphysema. With a clear conception of the normal structure and function of the bronchial tree in mind, it is apparent that complete bronchiolar obstruction (especially by mucus) is a frequent, although usually shortlived phenomenon, even in health. The close relation of bronchiolar obstruction to acute bronchiolitis is emphasized. In every respiratory tract infection, diffuse and relatively prolonged obstruction and inflammation of the smallest bronchioles occur. Such infections are not without sequelae; permanent damage is frequent and bronchiolar obliteration is not the rare phenomenon it has been considered in the past. It occurs (following obstruction and inflammation) in aerated as well as non-aerated lung tissue, aeration being maintained by collateral ventilation. Although collateral ventilation is remarkably efficient, with sufficiently extensive obstruction of bronchioles air-trapping may result in the collaterally ventilated lung. This may distend and even disrupt passages immediately beyond the obstructed bronchioles (thus producing emphysema), particularly if the pressure of the trapped air is considerable, as with coughing. Hence primarily, emphysema is seen to be the result of mechanical forces exerted by air trapped beyond obstructed bronchioles, the duration of the obstruction being one factor determining the degree of change. The significance of permanent inflammatory obliteration is obvious, the severity of the emphysema being, in part, proportional to the number of bronchioles obliterated. Along with the effect of airtrapping, in the exudative phase of bronchiolitis, alterations in the elasticity of the walls of the dilated passages add to the effect produced by the distending force. These are the basic factors involved. The morphological and clinical variants of the disease are due to the interplay of numerous other factors which are considered in the section dealing with etiology. In this framework, the relation of chronic bronchitis to emphysema becomes apparent.