Although fungus-related sinusitis has been described for at least 2 centuries, a more detailed pathologic description of the problem as it relates to eosinophilic disease was not detailed until 1983, when "allergic fungal sinusitis" was described histopathologically. Until then, most fungal sinus disease was perceived to occur in immunosuppressed diabetic patients with invasive fungus. It is now acknowledged that depending upon the immune status of the host, fungus-related sinus disease can take several forms. Interest in this subject matter was intensified in 1999, when it was suggested that fungi might be an important cause of most cases of chronic rhinosinusitis. This hypothesis remains controversial, and there is mounting evidence to support the multifactorial nature of chronic rhinosinusitis, which may include fungus. In fact, etiologic factors for all forms of fungus-related sinus disease are still poorly understood. The prevalence of the disease and the dominant fungal pathogen appear to vary in different geographic regions and probably are related to individual host conditions. Immunoglobulin E-mediated allergic reactions to mold appear to be associated with disease in some patients, but not in all. Although antifungal therapy is known to be lifesaving for invasive disease, its role in extramucosal disease is less well defined. Preliminary trials suggest that some systemic and topical antifungal agents are of clinical benefit in extramucosal disease. Since sinus fungi are rarely invasive in immunocompetent individuals, it is not clear whether the effects of the antifungal treatments are a result of the antifungal action itself, or due to additional properties these drugs possess. This review summarizes the available data and presents some of our clinical and experimental findings as to the role of fungus in chronic rhinosinusitis.