Many slime molds, mainly the 'cellular' slime molds, do not spend most of their time in this state. Most slime molds are smaller than a few centimeters, but some species may reach sizes up to several square meters and masses up to 20 kilograms. This is mostly seen with the Myxogastria, which are the only macroscopic slime molds. Their common name refers to part of some of these organisms' life cycles where they can appear as gelatinous 'slime'. More than 900 species of slime mold occur globally. Although not forming a single monophyletic clade, they are grouped within the paraphyletic group referred to as kingdom Protista.

Slime molds were formerly classified as fungi but are no longer considered part of that kingdom. Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms that can live freely as single cells, but can aggregate together to form multicellular reproductive structures. Mycetozoa from Ernst Haeckel's 1904 Kunstformen der Natur ( Artforms of Nature)