Allergic bronchopulmonary aspergillosis (ABPA) is a complex clinical entity that results from an allergic immune response to Aspergillus fumigatus, most often occurring in a patient with asthma or cystic fibrosis. Sensitization to aspergillus in the allergic host leads to activation of T helper 2 lymphocytes, which play a key role in recruiting eosinophils and other inflammatory mediators. ABPA is defined by a constellation of clinical, laboratory, and radiographic criteria that include active asthma, serum eosinophilia, an elevated total IgE level, fleeting pulmonary parenchymal opacities, bronchiectasis, and evidence for sensitization to Aspergillus fumigatus by skin testing. Specific diagnostic criteria exist and have evolved over the past several decades. Staging can be helpful to distinguish active disease from remission or end-stage bronchiectasis with progressive destruction of lung parenchyma and loss of lung function. Early recognition allows treatment with corticosteroids, which are effective but may be required indefinitely. There is some evidence to support the use of newer antifungal azoles as corticosteroid-sparing agents. Patients must be followed closely for recurrent disease. ABPA should be considered in all patients with asthma or cystic fibrosis, but especially in those with difficult to control disease.