Kingdom Fungi is limited to eukaryotes that form chitinous, resistant propagules (fungal spores) and chitinous cell walls and that lack undulipodia (that is, are amastigote, or immotile) at all stages of their life cycle. Of the 1,500,000 species of fungi estimated to exist, about 60,000 have been described; most are terrestrial, although a few truly marine species are known. Because fungi often differ only in subtle characteristics, such as the details of structure, pigments, and complex organic compounds, it is likely that many have not yet been recognized as distinct species.

Fungi were traditionally aligned with plants, and some classification schemes formerly considered the fungi to be a subkingdom of Kingdom Plantae. However, fungi are clearly more closely related to animals than to plants, considering that chitin is the main component both of fungal cell walls and of the arthropod exoskeleton (Phyla Chelicerata through Crustacea). In comparison, plant cell walls instead contain cellulose, a polysaccharide similar to chitin. At any rate, fungi differ from animals and plants in life cycle, in mode of nutrition, in pattern of development, and in many other ways. We thus support here the many mycologists who feel that fungi constitute their own kingdom.