Branchiopoda is a group of primitive and primarily fresh water crustaceans, mostly resembling shrimp. There are over 900 known species worldwide. A few are well-known, including Artemia (brine shrimp, called Sea-Monkeys when marketed as novelties), and Daphnia, both of which are raised as aquarium food or as interesting pets in and of themselves. Branchiopods should not be confused with Brachiopods.

In older classifications, the superorder Diplostraca was made up of the two orders Conchostraca and Cladocera. According to newer data, however, these orders may be artificial (at least in their present state) as some of their members seem to have a paraphyletic origin within the Diplostraca. For this reason, the order Conchostraca is no longer used.

Although a few species of water flea have adapted to a life in the sea, these crustaceans belong primarily in fresh water. The oldest known species is a 500 million years old fossil fairy shrimp called Rehbachiella kinnekullensis. Fairy shrimps are also seen as the most original and primitive members of the class. Branchiopods in general are equipped with a ventral food groove, useful for suspension and filter feeding. The water current in the ventral food groove, used for breathing and feeding in most species, are producing by a battery of unspecialised legs. This is thought to resemble a very original way of living among the crustaceans.