This chapter discusses physiology of the nose and paranasal sinuses, where the minimal cross-sectional area of the nasal cavity, i. e. the radius of the tube through which the air is flowing, is the most important factor determining nasal resistance and airflow. Dynamic collapse may also be noted at the internal nasal valve causing an increase in resistance. The nasal mucosa is innervated by both the trigeminal nerve and the autonomic nervous system. The maxillary nerve exits the foramen rotundum at the skull base, enters the pterygopalatine fossa, and branches into the greater palatine nerve, the nasopalatine nerve, the zygomatic nerve and the alveolar branches. The autonomic nervous system is made up of both parasympathetic and sympathetic nerve fibers. The nasovascular reflex causes peripheral vasoconstriction with nasal stimulation and nasocardiac reflex results in bradycardia and hypotension during nasal manipulation. Mucosal immunity can be broadly classified as either adaptive or innate.