During embryonic development, induction of bone formation is spatially and temporally regulated to form the skeletal elements. After birth, new bone formation is normally limited to bone regeneration during fracture repair, a process that is also precisely regulated. Induction of extraskeletal bone formation, also known as heterotopic ossification (HO), is a pathological condition in which bone forms in tissues outside of the normal skeleton. Heterotopic ossification forms through endochondral or intramembranous processes and produces qualitatively normal bone tissue. Extensive heterotopic ossification that begins during childhood occurs in two clinically distinct genetic diseases: fibrodysplasia ossificans progressiva (FOP) and progressive osseous heteroplasia (POH). Extraskeletal bone formation also can occur in cases other than these rare genetic diseases; such conditions are referred to as nonhereditary heterotopic ossification (NHHO). In adults, heterotopic ossification is a frequent complication of a number of common conditions associated with severe tissue trauma, such as spinal cord and head injuries, hip replacement surgery, severe burns, and high impact war wounds, as well as in age-associated conditions such as atherosclerosis and pressure ulcers.