The Percopsiformes are a small order of ray-finned fishes, comprising the trout-perch and its allies. It contains just ten extant species, grouped into seven genera and three families. Five of these genera are monotypic They are generally small fish, ranging from 5 to 20 cm (2.0 to 7.9 in) in adult body length. They inhabit freshwater habitats in North America. They are grouped together because of technical characteristics of their internal anatomy, and the different species may appear quite different externally.