Maxillopoda is a diverse class of crustaceans which includes the barnacles, copepods and a number of related animals.

It does not appear to be a monophyletic group, and no single character unites all the members.^[2]

With the exception of some barnacles, maxillopodans are mostly small,^[3] including the smallest known arthropod, Stygotantulus stocki.^[2] They often have short bodies, with the abdomen reduced in size, and generally lacking any appendages^[3] This may have arisen through paedomorphosis.^[3]

Apart from barnacles, which use their legs for filter feeding, most maxillopodans feed with their maxillae. Their bodyplan has 5 head segments, 6 thoracic segments and 4 abdominal segments, followed by a telson (tailpiece).^[4]

The fossil record of the group extends back into the Cambrian, with fossils of barnacles and tongue worms known from that period.^[5][6]

Six subclasses are generally recognised, although many works have included the ostracods among the Maxillopoda.^[2] Of the six groups, only Mystacocarida are entirely free-living; all the members of the Tantulocarida, Pentastomida and Branchiura are parasitic, and many of the Copepoda and Thecostraca are parasites.