The classification of cystic lesions of the lung remains problematic, as attempts at classification based on size of cysts do not satisfactorily address pathogenesis and do not easily accommodate lesions with overlapping features. In 1977, Stocker described three types of congenital cystic adenomatoid malformations (CCAM, types I-III) based on their gross appearance: type I (large, often multiloculated cysts >2 cm in diameter), type II (more uniform small cysts <2 cm), and type III (solid “adenomatoid” type, not grossly cystic). This classification has been helpful in pointing out the varied nature of developmental lesions of the lung and is useful as long as one recognizes that not all developmental cystic lesions of the lung can be fit into this classification. In fact, the type III lesion is now known to represent pulmonary hyperplasia. Of the two major types—large cyst (Stocker type I) and small cyst (Stocker type II)—the large cyst is more common and more easily recognized. It should be noted that the small cyst type is likely part of a malformation sequence resulting from airway obstruction during development. An obstructive lesion, most commonly bronchial atresia, is identified in virtually all cases with the microcystic pattern of maldevelopment, although the obstructing lesion may easily be overlooked (see Chapter 135). In cases in which an obstructive lesion is identified, the microcystic area should not be considered as a separate entity but rather as part of a malformation sequence. In contrast, the pathogenesis of the large cyst type remains uncertain, and it is considered a distinct malformation.