Bone is a hard substance that makes up the skeleton, which supports the body and provides protection for the organs. Bone is composed of minerals, mainly calcium and phosphate, which it stores and provides to the body as they are needed.

Bone consists of three layers: the outside covering of the bone (periosteum); the hard middle (compact) bone; and the inner spongy (cancellous) bone. The covering of the bone contains nerves and blood vessels that feed the hard bone. Holes and channels run through the hard bone to supply oxygen and nutrients to the inner bone cells. The spongy bone contains bone marrow, which produces red and white blood cells and platelets.

Normal bone is constantly dissolving and being absorbed into the body and then being rebuilt in a process called remodeling. This allows bones to react to changes in body weight and structure and to increase bone strength in areas of stress.