The world's smallest known animals only consist of a few hundred cells and measure as little as 45 micrometers in length (45 millionths of a meter). These microfauna include the smallest gastrotichs (<60 microns), nematodes (<80 microns), rotifers (<100 microns), micrognathozoans (<100 microns), the smallest mites, such as Aberoptus championus (<120 microns), parasitic wasps such as Dicopomorpha echmepterygis (137 microns, the smallest insect), the smallest beetles such as Scydosella musawasensis (300 microns), members of the phylum Cycliophora such as Symbion (347 microns), jaw worms (500 microns), and Trichoplax adhaerens (500 microns). Nematodes, rotifers, jaw worms, and micrognathozoans are aquatic, while mites, wasps, and beetles are terrestrial.

Most of the smallest animals are aquatic, partially because the layer of mud on the ocean floor is an excellent habitat for tiny organisms. The small invertebrates that live in the mud are known as the meiofauna, infauna, or the benthos. Aside from bacteria, they are probably among the most numerous animals on the planet (especially nematodes), found at modest densities in almost every square mile of the ocean floor, where they consume the endless organic "snow" that falls from above. The smallest animals are also among the most numerous here. They coexist with abundant bacteria, which they feast upon. Note that bacteria and other unicellular organisms are not considered the smallest animals because the definition of an animal is an organism that is by definition multicellular.

Sometimes, species named the "smallest animals" are not very small at all, such as fish of the genus , which can be as short as 8 mm. Aphids have a size between 1 and 10 mm, and are clearly visible to the naked eye. However, all the truly smallest animals are microscopic, as human vision cannot easily discern objects smaller than about 500 microns, and many animals can pass through a 500 micron filter. Many of the smallest animals are somewhat poorly studied due to their tiny size, but may hold the key to crucial debates of biological classification. For instance, it is thought that the tiny gastrotich, which may be a relative of nematodes, holds the key to the Edcysozoa hypothesis, over whether animals that shed a cuticle are all descendants of a common ancestor, or whether the quality arose independently in different lineages.