Aspergillus is a ubiquitous fungus. It is commonly isolated as an upper respiratory tract saprophyte and is the most frequent contaminant in laboratory specimens. Because species of aspergillus are omnipresent, one must be cautious in ascribing a causal role to the fungus obtained from patients. Aspergillus has low pathogenicity for humans and animals and rarely invades the immunologically competent host. Although the fungus can affect any organ system, the respiratory tract is involved in >90% of afflicted patients. Pulmonary aspergillosis, depending whether the host is atopic or immunosuppressed, may be classified under four categories: allergic or hypersensitivity syndromes, saprophytic colonization, invasive (infective) dissemination, and chemical or toxic pneumonitis. These entities differ clinically, radiologically, immunologically, and in their response to various therapeutic agents. An increased awareness, an aggressive approach to securing the diagnosis, and instituting early and appropriate therapy are needed to reduce the high morbidity and mortality caused by many of the aspergillus-related syndromes.