Our objective is to analyze and understand the physiology and patho-physiology of normal nasal breathing. To this end, airflow simulations based on computational fluid dynamics (CFD) are conducted for a highly detailed anatomy of the upper respiratory tract from the external nose deep into the trachea, including frontal and maxillary sinuses, as well as the ethmoid. Complex flow phenomena are investigated with regard to individual anatomy and its variations. We are aiming to gain insight into the relationship between morphology and flow behaviour in order to provide general treatment proposals and to develop computer assisted planning tools for decision support in functional rhinosurgery.