The anatomy of the paranasal sinuses is complex and variable, changing from patient to patient, and even from side to side in the same patient. This complexity results from the variable patterns of sinus ventilation and drainage dictated by independent sinus development. Sinus ventilation and drainage determines intrasinus mucociliary health, which also determines the efficiency of sinus growth and pneumatization. The final individual sinus configuration is additionally influenced by the evolving growth of neighboring sinuses, competing for space as they pneumatize the facial bones and skull base, also in competition with erupting teeth in the maxilla and the ever-constant intracranial pressure gradients along the skull base. The complex interaction of all these factors over time dictates an endless combination of anatomic variations that force the rhinologist to evaluate each side of the face as a completely independent anatomic, functional and surgical entity.