Nasal high flow (NHF) cannulae are used to deliver heated and humidified air to patients at steady flows ranging from 5-50 l/min. Knowledge of the airflow characteristics within the nasal cavity with NHF and during natural breathing is essential to understand the treatment's efficacy. In this thesis, the distribution and velocity of the airflow in the human nasal cavity have been mapped during natural and NHF assisted breathing with planar- and stereo-PIV in both steady and oscillatory flow conditions. Anatomically accurate transparent silicone models of the human nasal cavity were constructed using CT scan data and rapid prototyping. Breathing flowrates and waveforms were measured in vivo and dimensionally scaled by Reynolds and Womersley number matching to reproduce physiological conditions in vitro. Velocities of 2.8 and 3.8 m/s occurred in the nasal valve during natural breathing at peak expiration and inspiration, respectively; however on expiration the maximum velocity of 4.2 m/s occurred in the nasopharynx. Velocity magnitudes differed appreciably between the left and right sides of the nasal cavity, which were asymmetric. NHF modifies nasal cavity flow patterns significantly, altering the proportion of inspiration and expiration through each passageway and producing jets with in vivo velocities up to 20.8 m/s for 40 l/min cannula flow. The main flow stream passed through the middle airway and along the septal wall during both natural inspiration and expiration, whereas NHF inspired and expired flows remained high through the nasal cavity. Strong recirculating features are created above and below the cannula jet. Results are presented that suggest the quasi-steady flow assumption is invalid in the nasal cavity during both natural and NHF assisted breathing. The importance of using a three-component measurement technique when investigating nasal flows has been highlighted. Cannula flow has been found to continuously flush the nasopharyngeal dead space, which may enhance carbon dioxide removal and increase oxygen fraction. Close agreement was found between numerical and experimental results performed in identical conditions and geometries.