The lower the frontal approach to the anterior cranial fossa, the greater the likelihood of entering the frontal sinus. Like most of the paranasal sinuses, the frontal sinus undergoes most of its development in postnatal life (see Lang 1985 for details). The mean height of the sinus is 24.3 mm, with a range of 5–66 mm (Milosslawski 1903). In our material as well, we found a frontal sinus with a maximum vertical dimension of 60 mm (see Fig. 7 in Lang 1983). It should be noted that the frontal sinus enlarges in both the vertical and sagittal dimensions after middle age. The floor of the frontal sinus may be situated on, above, or below the plane of the cribriform plate (see Figs. 44 and 45 in Lang 1975, Fig. 69 in Lang 1983, and Fig. 7 in Lang 1983). The medial parts of the floor are generally set lower than the lateral parts. Occasionally there may be a third frontal sinus with its own orifice or a single frontal sinus with two orifices (see Fig. 69 in Lang 1983). The surgical approach to the medial part of the anterior cranial fossa (the olfactory fossa) relies on a knowledge of the torus olfactorius. Occasionally the olfactory fossa bulges into a prominently developed frontal sinus just adjacent to the midline. The frontal sinus also may be permeated by septa of varyious sizes. These septa may conduct vessels that establish anastomoses between the ophthalmic artery and the frontal branch of the middle meningeal. Rarely, rarefactions are observed in the orbital aspect of the floor of the frontal sinus (see Fig. 38 in Lang 1983).