An exoskeleton is an external, often hard, covering or integument that provides support and protection to the body of certain animals, notably arthropods and molluscs. In arthropods, it consists of a thick horny covering attached to the outside of the body and may be jointed and flexible. The exoskeleton does not grow as the animal grows; instead it is shed periodically and the animal generates a new one. See also endoskeleton.

The support of muscles for movement is a major function of the exoskeleton. The muscles are attached across a joint (A), as in vertebrates, but inside the skeleton. The contraction of one muscle of a pair and relaxing the other results in movement at the flexible joint. A cross-section through the body at a leg insertion (B) shows the part played by muscle attachment points and by the thin flexible cuticle at joints.