The fungi is a large group including yeasts, molds and mushrooms. They inhabit widely in aquatic and terrestrial environments. Fungi are mostly osmotrophic (but earliest-diverging members are phagotrophic), saprobic or parasitic. Many species live symbiotically with photosynthetic organisms (e.g. lichens, mycorrhizal fungi). Fungi are indispensable for human life. Many species are used as a direct source of foods (e.g. mushrooms and truffles), fermentation of various food products (e.g. wine, soy sauce) and sources for usuful material (e.g. antibiotics, vitamins). Furthermore, they perform an essential role in ecosystems in decomposing organic matter.

Fungi are unicellular (e.g. yeasts) or filamentous. The fungal filaments called hyphae are usually uniseriate multicellular, but multinuclear coenocytic in some species (e.g. 'Zygomycota'). The hyphae show apical growth. Some basidiomycetes and ascomycetes form macroscopic mycelia such as mushrooms. The cells are covered by cell wall composed of chitin and β-1,3-1,6-glucan. Mitochondrial cristae are usually flat. In some fungi, mitochondria lost cristae, genome and the abirity of aerobic respiration, and transformed into hydrogenosomes (Neocallimastigomycota) or mitosomes (Microsporidia). Fungi possess no flagella and centrioles except for 'chytridis' (sensu lato). Lysine is synthesized by the α-aminoadipate (AAA) pathway. Asexual reproduction by means of binary fission, budding, zoosporogenesis, sporogenesis etc. Some 'chytridis' (sensu lato) form flagellate gametes involved with isogamy to oogamy. Other fungi produce spores (zygospores, ascospores, basidiospores) via fusion of cells or hyphae. There is long dikaryotic interval between fusion of cells/hyphae and nuclear fusion in the Basidiomycota and Ascomycota (thus called Dikarya). Most fungi have haplontic life cycle but some species of the Blastocladiomycota show typical alternation of generations between haploid gametophyte and diploid sporophyte.

Slime molds (myxomycetes) and water molds (oomycetes) are distantly related to the true fungi (eumycetes). The true fungi are traditionally divided into the Chytridiomycota, Zygomycota, Basidiomycota and Ascomycota, but the former two are now considered to be non-monophyletic. Recent studies suggest that microsporidia traditionally classified into protozoa is a member of the Fungi.