This study has permitted some tentative conclusions. First, it has demonstrated the normal development of the ossification centers of the calcaneus and talus as seen roentgenographically in the posterior tangential view. In the neutral foot the posterior facet appears at the age of six months, and the sustentaculum tali appears between one and two years of age and is fully developed at four or five years. The superior surfaces of both these structures lie in the horizontal plane, and there is a definite step between them caused by the elevation of the sustentaculum tali above the subtalar joint. Second, differences have been demonstrated in the development of neutral, flat, and club feet. In the flat foot, the calcaneus is everted, the subtalar-joint surface is inclined medially, and the sustentaculum tali is lowered. The talus does not appear to be adequately supported. In the club foot, the calcaneus is inverted, the subtalar joint inclines in the reverse direction, and the sustentaculum tali is markedly elevated. Third, this study has provided additional roentgenographic criteria for the diagnosis of these deformities and indicated the usefulness of the posterior tangential view as an aid in such diagnosis.