This chapter discusses physiology of the nose which conditions the air and provides a defence against outside attack. Nasal neurovascular reflexes are important in initiating such basic mechanisms as sneezing and heat exchange. Viscosity is best envisaged by thinking of a moving column of gas as being composed of successive layers of material. Airflow simulations created either with laboratory models or by computer simulations suffer from the same disadvantages but due to the inaccessibility of the nasal cavity, are all that we currently have as realistic models of normal nasal performance. Non-invasive measurements performed with laser Doppler velocimeter during quiet breathing have also shown that airflow in the nose is mostly laminar and have suggested that flow is streamlined by the turbinates with greater velocities in the lower half of the cavity and near the septum. The nasal cavity performs majority of the airconditioning that equilibrates the inspired air with alveolar conditions.