The chapter briefly reiterates the different modes and the time line of bone formation in the human embryo. Most skeletal elements of modern vertebrates are traced to the endoskeleton (skeleton formed inside the body) found in primitive vertebrates. Vestiges of the exoskeleton (skeleton formed outside the body) are present in the skull and pectoral girdle. Bones derived from the primitive endoskeleton develop first into cartilage templates but are subsequently replaced by bone through endochondral ossification. The process of endochondral ossification also initiates with the aggregation of undifferentiated mesenchymal cells to form condensations. All primary ossification centers appear between weeks 7 and 12 of embryonic life in humans. Ossification centers in the neural arches appear in a cephalocaudal sequence except in the atlas and axis, in which ossification is slightly delayed. The sternum arises as a pair of cartilaginous bands that fuse along the midline as the ventral body wall develops. The primary ossification centers of the pectoral girdle appear before those of the pelvis in the following order: clavicle, scapula, ilium, ischium, and pubis.