Slime moulds are peculiar protists that normally take the form of amoebae, but under certain conditions develop fruiting bodies that release spores, superficially similar to the sporangia of fungi. Although cosmopolitan in distribution, they are usually small and rarely noticed. There are several different groups.

Most notable are the plasmodial slime moulds or myxogastrids, where the feeding stage takes the form of a giant amoeba with thousands of nuclei, called a plasmodium. The very largest reach areas of up to two square metres, the largest undivided cells known, and many have bright colors such as yellow, brown, and white. Under dry conditions they may form resting structures called sclerotia. Once produced, spores release biflagellate or amoeboid gametes, which fuse pairwise to produce new plasmodia.

The cellular slime moulds or dictyostelids take the form of individual amoebae, but under stress aggregate to form a multicellular assembly called a pseudoplasmodium or slug. This migrates to a new location, then forms into a fruiting body, usually with a stalk formed from dead amoebae. Spores release new amoebae. Similar life-cycles are found among the acrasids, now known to be an unrelated group, and among the myxobacteria.

There are also several uninucleate amoebae, called protostelids, that secrete stalks and develop into spores individually. It now appears that they gave rise to both the myxogastrids and dictyostelids, although they were considered unrelated based on rRNA. Comparison of protein genes support a close relationship between the three groups and place them among the Amoebozoa.

Slime moulds are common objects of study. Dictyostelids are used as examples of cellular communication and differentiation, and may provide insights into how multicellular organisms develop. Plasmodia are useful for studying cytoplasmic streaming. It has been observed that they can find their way through mazes by spreading out and choosing the shortest path, an interesting example of information processing without a nervous system.

Slime moulds were originally considered fungi by mycologists and amoebae by zoologists, respectively classified as Myxomycetes (slime fungi) or Mycetozoa (fungus animals). Both names are still used among different groups of specialists. Various other protists that form cellular aggregates such as acrasids, Labyrinthulomycetes, and plasmodiophorids are traditionally included, but the formal taxon is now often restricted to the true plasmodial slime moulds and their relatives.