An exoskeleton is an external skeleton that supports and protects an animals body, in contrast to an internal skeleton in humans. Some animals, such as turtles, have both an endoskeleton and an exoskeleton. Exoskeletons contain rigid and resistant components that fulfill a set of functional roles in many animals including protection, excretion, sensing, support, feeding, and acting as a barrier against desiccation in terrestrial organisms. Arthropod exoskeletons contain chitin, and the addition of calcium carbonate makes them harder and stronger, at the price of increased weight. Ingrowths of the arthropod exoskeleton known as apodemes serve as attachment sites for muscles. Exoskeletons have roles in defense from pests and predators and in providing an attachment framework for musculature.

Exoskeletons are not limited to animals. A robotic exoskeleton is a mechanical device worn by a human being for certain purposes or applications. Exoskeletons can be used to increase the users strength, improve physical endurance, and reduce the physical burden on soldiers. Most commonly, exoskeletons are used in the health sector and, in particular, in rehabilitation medicine.

Examples of exoskeletons within animals include the arthropod exoskeleton shared by chelicerates, myriapods, crustaceans, and insects, as well as the shell of certain sponges and the mollusc shell shared by snails, clams, tusk shells, chitons, and nautilus. Some creatures have a hard outer covering on their bodies, which is called an exoskeleton.