Fungi are eukaryotic, unicellular organisms that are abundant in nature but rarely cause disease. Fungi are larger and genomically more complex than bacteria. Tubular aggregates of fungal cells are called hyphae, and if the hyphae show constrictions, they are called pseudohyphae. Discrete fungal cells are called yeast or spores, and spore-forming fruiting bodies are known as conidia or sporangia (Table 14-1). The diagnosis of pulmonary fungal infections depends on the demonstration of fungi in tissue and the confirmation of species by culture. Fungi can infect healthy individuals, although most infections by far are seen in an immunocompromised host. Infection occurs with inhalation of the airborne fungal hyphae, resulting in an exudative or a granulomatous response. Chronic infection can cause fibrosis with bronchiectasis and distortion of lung architecture. Transbronchial biopsy can be useful in the diagnosis of many fungal infections, including histoplasmosis, cryptococcosis, coccidioidomycosis, candidiasis, and aspergillosis. Some fungi are visible on hematoxylin and eosin stains; however, special stains such as Gomori methenamine silver (GMS), Grocott silver stain, and periodic acid-Schiff (PAS) stains are often needed for screening and confirmation. Fungi may be seen in the lung as yeast forms, spherules, hyphae, or pseudohyphae. A diagnosis of a particular fungus can be made based on the morphology and size.