Characiformes is an order of ray-finned fish, comprising the characins and their allies. Grouped in 18 recognized families, there are more than two thousand different species, including the well-known piranha and tetras.
Temporal range: Early Cretaceous–Recent 
Characiformes form part of a series called Otophysi within the superorder Ostariophysi. Otophysi contains three other orders, Cypriniformes, Siluriformes, and Gymnotiformes. Characiformes forms a group known as Characiphysi with Siluriformes and Gymnotiformes. Characiformes is the sister group to the orders Siluriformes and Gymnotiformes, though this has been debated in light of recent molecular evidence.
Originally the characins were all grouped within a single family, the Characidae. Since then 18 different families have been separated out. However, classification varies somewhat, and the most recent (2011) study confirms the circumscribed Characidae as monophyletic. Currently, there are eighteen families, about 270 genera, and at least 1674 species. The suborder Citharinoidei, which contains the families Distichodontidae and Citharinidae, is considered the sister group to the rest of the characins, suborder Characoidei.
The oldest characiform is Santanichthys of the early Cretaceous (Albian stage) of Brazil. While all extant species are freshwater, this species was probably either brackish or marine. Many other fossils are also known. Characiformes likely first diversified during the Cretaceous period, though fossils are poorly known. During the Cretaceous period, the rift between South America and Africa would be forming; this may explain the contrast in diversity between the two continents. Their low diversity in Africa may explain why some primitive fish families and Cypriniformes coexist with them while they are absent in South America, where these fish may have been driven extinct. The characiforms had not spread into Africa soon enough to also reach the land bridge between Africa and Asia. The earliest they could have spread into Central America was the late Miocene.
Characins possess a Weberian apparatus, a series of bony parts connecting the swim bladder and inner ear. Superficially, the Characiformes somewhat resemble their relatives of the order Cypriniformes, but have a small fleshy adipose fin between the dorsal fin and tail. Most species have teeth within the mouth, since they are often carnivorous. The body is almost always covered in well-defined scales. The mouth is also usually not truly protractile.
The largest characin are Hydrocynus goliath and Salminus franciscanus, both of which are up to 1.3 metres (4.3 ft). The smallest size is about 1.7 centimetres (0.67 in) in the Bolivian pygmy blue characin, Xenurobrycon polyancistrus. Many members are under 3 centimetres (1.2 in).
Characins are most diverse in the Neotropics, where they are found in lakes and rivers throughout most of South and Central America. The red-bellied piranha, a member of the Serrasalmidae family within Characiformes, is endemic to the Neotropic ecozone. At least 209 species of characins are found in Africa, including the distichodontids, citharinids, alestiids, and hepsetids. The rest of the characins originate from the Americas.
A few characins become quite large, and are important as food or game. Most, however, are small shoaling fish. Many species known as tetras are popular in aquaria thanks to their bright colors, general hardiness, and tolerance towards other fish in community tanks.