Arthropods (phylum Arthropoda) (from Greek ἀρθρον, meaning joint and πούς/ποδός, meaning foot) are the largest phylum of animals and include the insects, arachnids, crustaceans, and others. Approximately eighty percent of extant animal species are arthropods, with over a million modern species described and a fossil record reaching back to the early Cambrian. Arthropods are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various symbiotic and parasitic forms. They range in size from microscopic plankton (~0.25 mm) up to forms several metres long.

Arthropods are characterised by the possession of a segmented body with appendages on each segment. They have a dorsal heart and a ventral nervous system. All arthropods are covered by a hard exoskeleton that is made out of chitin, a polysaccharide. Periodically, an arthropod sheds this covering when it moults. This covering makes arthropods less prone to dehydration.