Arthropods form the largest clade of animals, originating in the early Cambrian and diversifying into a wide array of marine, terrestrial, and freshwater forms. They are united by a suite of defining traits: a segmented body plan with specialized tagmata, jointed appendages, and a chitinous exoskeleton that is periodically shed through ecdysis. Their nervous system features a dorsal brain and paired ventral nerve cords, and most possess compound eyes or ocelli.

The group includes extinct trilobites as well as four major extant clades: Chelicerata (spiders, horseshoe crabs), Myriapoda (millipedes, centipedes), Crustacea (crabs, shrimp), and Hexapoda (insects). Arthropods differ from close relatives like annelids by having articulated limbs and a hardened cuticle, and from other ecdysozoans such as onychophorans by their pronounced tagmosis and rigid exoskeleton. Their fossil record spans over 500 million years and preserves both stem and crown group representatives, illustrating the deep and varied evolutionary history of the clade.