A skeleton is the structural frame that supports the body of most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal frame to which the organs and soft tissues attach; and the hydroskeleton, a flexible internal structure supported by the hydrostatic pressure of body fluids.

Vertebrates are animals with an endoskeleton centered around an axial vertebral column, and their skeletons are typically composed of bones and cartilages. Invertebrates are other animals that lack a vertebral column, and their skeletons vary, including hard-shelled exoskeleton (arthropods and most molluscs), plated internal shells (e.g. cuttlebones in some cephalopods) or rods (e.g. ossicles in echinoderms), hydrostatically supported body cavities (most), and spicules (sponges). Cartilage is a rigid connective tissue that is found in the skeletal systems of vertebrates and invertebrates.

The term skeleton comes from Ancient Greek σκελετός (skeletós) 'dried up'. Sceleton is an archaic form of the word.

Skeletons can be defined by several attributes. Solid skeletons consist of hard substances, such as bone, cartilage, or cuticle. These can be further divided by location; internal skeletons are endoskeletons, and external skeletons are exoskeletons. Skeletons may also be defined by rigidity, where pliant skeletons are more elastic than rigid skeletons. Fluid or hydrostatic skeletons do not have hard structures like solid skeletons, instead functioning via pressurized fluids. Hydrostatic skeletons are always internal.

An exoskeleton is an external skeleton that covers the body of an animal, serving as armor to protect an animal from predators. Arthropods have exoskeletons that encase their bodies, and have to undergo periodic moulting or ecdysis as the animals grow. The shells of molluscs are another form of exoskeleton. Exoskeletons provide surfaces for the attachment of muscles, and specialized appendanges of the exoskeleton can assist with movement and defense. In arthropods, the exoskeleton also assists with sensory perception.

An external skeleton can be quite heavy in relation to the overall mass of an animal, so on land, organisms that have an exoskeleton are mostly relatively small. Somewhat larger aquatic animals can support an exoskeleton because weight is less of a consideration underwater. The southern giant clam, a species of extremely large saltwater clam in the Pacific Ocean, has a shell that is massive in both size and weight. Syrinx aruanus is a species of sea snail with a very large shell.

Endoskeletons are the internal support structure of an animal, composed of mineralized tissues, such as the bone skeletons found in most vertebrates. Endoskeletons are highly specialized and vary significantly between animals. They vary in complexity from functioning purely for support (as in the case of sponges), to serving as an attachment site for muscles and a mechanism for transmitting muscular forces. A true endoskeleton is derived from mesodermal tissue. Endoskeletons occur in chordates, echinoderms, and sponges.

Pliant skeletons are capable of movement; thus, when stress is applied to the skeletal structure, it deforms and then regains its original shape. This skeletal structure is used in some invertebrates, for instance in the hinge of bivalve shells or the mesoglea of cnidarians such as jellyfish. Pliant skeletons are beneficial because only muscle contractions are needed to bend the skeleton; upon muscle relaxation, the skeleton will return to its original shape. Cartilage is one material that a pliant skeleton may be composed of, but most pliant skeletons are formed from a mixture of proteins, polysaccharides, and water. For additional structure or protection, pliant skeletons may be supported by rigid skeletons. Organisms that have pliant skeletons typically live in water, which supports body structure in the absence of a rigid skeleton.