The process by which bones form, beginning in the eighth week of an embryo's development, is known as osteogenesis or ossification. The majority of the skull bones and the clavicle develop through intramembranous ossification, in which the bones form directly from an embryonic tissue known as mesenchyme. All other bones of the body develop through endochrondral ossification, in which the bones are first formed in hyaline cartilage which then is gradually replaced by bone tissue. Endochrondral ossification of long bones begins in the diaphysis, or shaft, of the bone; the last stage is the ossification of the epiphyses, or the ends. This last stage occurs around the time of birth. Ossification continues throughout life, with the epiphyses finally fusing to the diaphysis in adulthood. Until then, a section of hyaline cartilage known as the epiphyseal plate remains between the diaphysis and the epiphysis.

After birth, increase in the length of bone is caused by growth of hyaline cartilage which is then replaced by bone tissue. Increase in the width of bone is caused by appositional growth, in which bone tissue is added to the surface of the existing bone. This new bone tissue is secreted from the deep layer of the periosteum, a connective tissue membrane that covers the bone.

The longest bone in the human body is the femur, sometimes called the thighbone.