Ossification is the process of bone formation, in which connective tissues, such as cartilage are turned to bone or bone-like tissue. The ossified tissue is invaginated with blood vessels. These blood vessels bring minerals like calcium and deposit it in the ossifying tissue. It is thought that this process led to bone as a structural element in vertebrates. Minerals were deposited in cartilage, which was used for storage. Bone was thus an exaptation from the ossified cartilage.

• Endochondral ossification: formation of bone from cartilage
• Intramembranous ossification: formation of bone from mesenchyme, esp. flat bones found in the skull
• Heterotopic ossification : formation of bone in extraskeletal soft tissue, esp. in connective tissue or muscle