Amoebozoa is a major taxonomic group containing about 2,400 described species of amoeboid protists, often possessing blunt, fingerlike, lobose pseudopods and tubular mitochondrial cristae. In most classification schemes, Amoebozoa is ranked as a phylum within either the kingdom Protista or the kingdom Protozoa. In the classification favored by the International Society of Protistologists, it is retained as an unranked " supergroup" within Eukaryota. Molecular genetic analysis supports Amoebozoa as a monophyletic clade. Most phylogenetic trees identify it as the sister group to Opisthokonta, another major clade which contains both Fungi and Animals as well as some 300 species of unicellular protists. Amoebozoa and Opisthokonta are sometimes grouped together in a high-level taxon, variously named Unikonta, Amorphea or Opimoda.
Amoebozoa includes many of the best-known amoeboid organisms, such as Chaos, Entamoeba, Pelomyxa and the genus Amoeba itself. Species of Amoebozoa may be either shelled (testate), or naked, and cells may possess flagella. Free-living species are common in both salt and freshwater, as well as soil, moss and leaf litter. Some live as parasites or symbiotes of other organisms, and some are known to cause disease in humans and other organisms.
While the majority of amoebozoan species are unicellular, the group also includes several varieties of slime molds, which have a macroscopic, multicellular stage of life during which individual amoeboid cells aggregate to produce spores.
Amoebozoa vary greatly in size. Some are only 10–20 μm in diameter, while others are among the largest protozoa. The well-known species Amoeba proteus, which may reach 800 μm in length, is often studied in schools and laboratories as a representative cell or model organism, partly because of its convenient size. Multinucleate amoebae like Chaos and Pelomyxa (the so-called "giant amoebae") may be several millimetres in length, and some multicellular amoebozoa, such as the "dog vomit" slime mold Fuligo septica, can cover an area of several square meters.