Postmortem studies of ten normal full-term infants and of three children, seven, nine, and fourteen years old, showed that the acetabular cartilage complex is a unit that is triradiate medially and cup-shaped laterally and is interposed between the ilium, ischium, and pubis. This complex is composed of epiphyseal growth-plate cartilage adjacent to these bones, of articular cartilage adjacent to these bones, of articular cartilage around the acetabular cavity, and, for the most part, of hyaline carilage. Interstitial growth within the triradiate part of the cartilage complex causes the hip socket to expand during growth. The concavity of the acetabulum develops in response to the presence of the spherical femoral head. The depth of the acetabulum increased during development as the result of interstitial growth in the acetabular cartilage, of appositional growth at the periphery of this cartilage, and of periosteal new-bone formation at the acetabular margin. At puberty, three secondary centers of ossification appear in the hyaline cartilage surrounding the acetabular cavity. These centers are homologous with other epiphyses in the skeleton. The os acetabuli, which is the epiphysis of the os pubis, forms the anterior wall of the acetabulum. The epiphysis of the ilium, which has been called the acetabular epiphysis, forms a good part of the superior wall of the acetabulum. A small epiphysis of the ischium was seen in the oldest patient, who was fourteen years old. The bone in these epiphyses expands toward the periphery of the acetabulum and thus contributes to its increase in depth.