Copepods are small aquatic crustaceans and are one of the most numerous metazoan groups in aquatic communities. Copepods inhabit a huge range of salinities, from fresh water to hypersaline conditions, and they can be found virtually everywhere there is water; from subterranean caves to pools collected in bromeliad leaves or in damp leaf litter on the ground, from streams, rivers, and lakes to the open ocean and the sediment layers beneath. Their habitats range from the highest mountain lakes to the deepest ocean trenches and from the cold polar ice-water interface to the hot active hydrothermal vents. Copepods may be free-living, symbiotic, or internal or external parasites on almost every major metazoan phylum. Adults typically have a body length in the 1-2 mm range, but adults of free-living species may be as short as 0.2 mm or as long as 17 mm. In the case of parasitic forms on large vertebrate hosts, body lengths may exceed 20 cm. Ecologically the planktonic copepods provide functionally important links in the aquatic food chain feeding on the microscopic algal cells of the phytoplankton and, in turn, being eaten by juvenile fish and other planktivores, including some whales. In fresh water copepods have the potential to act as a biological control mechanism for malaria by consuming mosquito larvae. However, they also serve as intermediate hosts of many animal parasites and even parasites of humans, including the fish tapeworm and guineaworm.

Although they belong to a separate class of crustaceans, Branchiura (commonly referred to as fish lice) are dealt with here along with the Copepoda, since many copepod researchers also study these external parasites of fish and amphibians. Most live in freshwater but about a quarter of species are marine. Together the Copepoda and Branchiura comprise over 200 described families; 2,600 genera and over 21,000 described species (both valid and invalid, including senior and junior synonyms).