Sinuses are mucosa-lined airspaces within the bones of the face and skull. The frontal sinuses are situated behind the superciliary arches.

The frontal sinuses are absent at birth, but are generally fairly well developed between the seventh and eighth years, only reaching their full size after puberty. The frontal bone is membranous at birth and there is rarely more than a recess until the bone tissue starts to ossify about age two. Consequently this structure does not show on radiographs before that time. Frontal sinuses are rarely symmetrical and the septum between them frequently deviates to one or other side of the middle line. Sinus development begins in the womb, but only the maxillary and ethmoid sinuses are present at birth. Approximately 5% of people have absent frontal sinuses.

Their average measurements are as follows: height 28 mm, breadth 24 mm, depth 20 mm, creating a space of 6-7 ml.

Each opens into the anterior part of the corresponding superior meatus of the nose through the frontonasal duct which traverses the anterior part of the labyrinth of the ethmoid. These structures then open into the hiatus semilunaris in the middle meatus.