Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typically around 1 mm (0.04 in) in size, but varying from 0.2 to 30 mm (0.008 to 1 in) in the case of Gigantocypris. Their bodies are flattened from side to side and protected by a bivalve-like, chitinous or calcareous valve or "shell". The hinge of the two valves is in the upper (dorsal) region of the body. Ostracods are grouped together based on gross morphology. While early work indicated the group may not be monophyletic and early molecular phylogeny was ambiguous on this front, recent combined analyses of molecular and morphological data found support for monophyly in analyses with broadest taxon sampling.

Ecologically, marine ostracods can be part of the zooplankton or (most commonly) are part of the benthos, living on or inside the upper layer of the sea floor. While Myodocopa is restricted to marine environments, the Podocopida are also common in fresh water, and terrestrial species of Mesocypris are known from humid forest soils of South Africa, Australia and New Zealand. But all three major podocopid lineages, Cypridocopina, Darwinulocopina and Cytherocopina, have several representatives living in terrestrial habitats. They have a wide range of diets, and the group includes carnivores, herbivores, scavengers and filter feeders.

As of 2008, around 2000 species and 200 genera of nonmarine ostracods are found. However, a large portion of diversity is still undescribed, indicated by undocumented diversity hotspots of temporary habitats in Africa and Australia. Of the known specific and generic diversity of nonmarine ostracods, half (1000 species, 100 genera) belongs to one family (of 13 families), Cyprididae. Many Cyprididae occur in temporary water bodies and have drought-resistant eggs, mixed/parthenogenetic reproduction, and the ability to swim. These biological attributes preadapt them to form successful radiations in these habitats.