An osteon, also known as a Haversian system, is a fundamental functional unit of much compact bone. It is a cylindrical structure that is typically between 0.25 mm and 0.35 mm in diameter and several millimeters in length. Osteons are formed during the process of bone remodeling or renewal, and they are also present in new bone. Each osteon consists of concentric layers of compact bone tissue called lamellae that surround a central canal, the Haversian canal, which contains the bones blood supply. The boundary of an osteon is the cement line. Osteons are not found in spongy bone, which is composed of plate or rod-shaped lamellae called trabeculae.

Osteons are dynamic bony structures, and their number, structure, and activity change over time in response to external stimuli. At a cellular level, osteons contain osteoblasts, which are bone-forming cells, osteoclasts, which resorb or break down bone, and osteocytes, which are mature bone cells. An equilibrium between osteoblasts and osteoclasts maintains bone tissue.

In bioarchaeological research and forensic investigations, osteons in a bone fragment can be used to determine the sex of an individual and age, as well as aspects of taxonomy, diet, health, and motor history. Osteons and their arrangement vary according to taxon, so that genus and sometimes species can be differentiated using a bone fragment not otherwise identifiable. However, examination of osteons is not of primary use in the analyses of osteological remains.