Cladocera are a diverse group of small crustaceans common in aquatic habitats, ranging from shallow temporary ponds to deep lakes and large rivers. Some groups (e.g., Podonidae) are widespread in the oceans. With the exception of predaceous onychopods and Leptodora, cladocerans consume algae and microbes, either grazing with filtering limbs from the water column or scraping periphyton off surfaces. Cladocerans serve as an important food for small fish, aquatic insects, and other zooplankton. Maximum body size ranges across two orders of magnitude, affecting a variety of physiological and ecological characteristics. The small size and flattened shape of most cladocerans allow rapid diffusion of gases and solutes between the environment and internal spaces, minimizing the need for specialized vascular systems. Most cladocerans reproduce by cyclic parthenogenesis, alternating long periods of asexual reproduction with infrequent sex, and resting egg formation. Populations are dominated by females and can grow rapidly when the conditions are suitable. Although resting eggs and asexual reproduction provide mechanisms that should favor dispersal, long distance migration is ordinarily quite rare. While some species are cosmopolitan, most cladocerans are restricted to single continents and recent invasions appear linked to human-mediated introductions.