Embryology is of importance to the surgeon both for the study of human developmental anatomy and for the analysis of congenital conditions resulting from malformed or arrested development. The embryonic development of the nose, and especially of the paranasal sinuses, is not yet fully understood. This histologic study of 23 fetal heads aged from 8 to 40 weeks of gestation demonstrates that all four pairs of paranasal sinuses are developed from the cartilaginous nasal capsule. The outpouching of the nasal mucous membranes is only a secondary phenomenon, rather than the primary force. This observation helps to elucidate the following clinical observations: (1) the association of maxillary sinus hypoplasia with hypoplasia of the uncinate process, (2) the origin of chondrosarcoma of the maxillary bone, and (3) pneumatization of the paranasal sinuses.