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Arthropods are a diverse phylum of invertebrate animals characterized by segmented bodies, an external exoskeleton of chitin, and jointed appendages. Their bodies are often divided into tagmata such as a head, thorax, and abdomen. They grow by molting as the rigid exoskeleton is shed and replaced. Arthropods have an open circulatory system with hemolymph, a ventral nerve cord, and sensory organs including antennae and compound eyes.

The phylum includes several major groups, notably insects (Hexapoda), arachnids (spiders, scorpions, and relatives), crustaceans (crabs, lobsters, shrimp, and many planktonic forms), and myriapods (centipedes and millipedes).

Anatomical diversity is vast, but all share segmented bodies and jointed limbs. Insects typically have six legs and often undergo metamorphosis, while crustaceans show a wide range of body plans and may have two pairs of antennae. Myriapods have numerous legs and elongated bodies, and arachnids generally have four pairs of legs and no antennae.

Ecologically, arthropods occupy nearly every habitat and play essential roles as pollinators, decomposers, herbivores, predators, and prey. They are central to many ecosystems and support human economies through pollination, food sources, pest control, and biomass.

The fossil record extends back to the Cambrian, highlighting an early burst of diversification. Exoskeletons aided protection and allowed colonization of diverse environments, with modern lineages continuing to adapt to changing conditions. Conservation and sustainable management concerns focus on habitat loss, climate change, and overharvesting affecting some arthropod groups.