As the population of patients with prolonged neutropenia, those receiving hematopoietic stem cell transplantation or lung transplantation, or those with human immunodeficiency virus or acquired immunodeficiency syndrome continues to increase, the rising incidence of pulmonary aspergillosis is unlikely to diminish. Aspergillus species are ubiquitous in nature, have no geographic predilection, and the spectrum of disease they cause is myriad, ranging from noninvasive disease with colonization to disseminated disease with an associated high mortality rate. The extent of disease is thus largely responsible for both the choice and the duration of antifungal therapy. Recent years have seen an expansion of antifungal agents, with efficacy against Aspergillus requiring an understanding of the full spectrum of disease for them to be used appropriately. Diagnosis is often difficult because existing tests lack desired sensitivity or specificity.