Fungi are key players in the health, diversity, and productivity of forest ecosystems in Pacific Northwest forests, as mycorrhizal associations, pathogens, decomposers, nontimber resources, and food resources for wildlife. A number of invertebrate species are associated with wood decay fungi, serve as vectors for fungal pathogens, or are fungivorous (consume fungi) and influence rates of wood decay and nutrient mineralization. In Washington and Oregon, 31 wildlife species among 8 families are fungivores, and at least 14 wildlife species disperse fungi. Down wood can provide nurse substrates for seedlings and beneficial mycorrhizal fungi, refuges from pathogenic soil fungi, sources of nutrients for decay fungi, and substrates supporting overall fungal diversity. Presence, density, distribution, and diversity of fungi are influenced by forest stand management practices, forest age class, and effects of fire. Old forests provide for a suite of rare fungi species. Old legacy trees retained during forest harvest can provide some degree of conservation of beneficial and rare fungi. Fungi can be difficult to detect and monitor; surveying for fungi at various times of the year, for multiple (at least 5) years, and by including hypogeous (belowground) samples, can improve detection rates. Studies are needed in the Pacific Northwest to quantify the amount of down wood—number of pieces, sizes, total biomass, percentage of forest floor cover, and other attributes—necessary for maintaining or restoring fungal biodiversity and viable levels of individual fungi species, especially rare species.