This article provides an overview of the major pathologic manifestations of allergic bronchopulmonary aspergillosis; patient characteristics; clinical, radiographic, and laboratory features of the disease; and current knowledge about its pathogenesis. Although allergic bronchopulmonary aspergillosis is an infrequent complication of asthma or cystic fibrosis, recognition of this disorder is important to avoid progression of bronchiectasis and lung parenchymal damage. Clinical, laboratory, and radiographic criteria allow for diagnosis of most cases, but the pathologist may encounter clinically unsuspected or atypical cases that require morphologic confirmation.