Nowadays various studies utilizing computational techniques are trying to contribute to the medical world. In this light Computational Fluid Dynamics (CFD) techniques are already used for some time to simulate flow behavior in the human airway system. This was mostly done for idealized and simplified airway models as it is only recently that truly realistic geometry models are made available. It is believed that improving CFD techniques with respect to these realistic lung geometries may improve medical practice and help lung surgeons. In this thesis the Reynolds Averaged Navier Stokes (RANS) equations are used to simulate airflow in a realistic human airway model derived from Computed Tomography (CT) scan data ranging up to 7 generations. This is done for inhalation at three different breathing rates. Due to the transitional nature of the flow, the _-_ SST model is chosen to predict turbulence. For solving the flow equations the Fine/Hexa software (Numeca, Brussels, Belgium) is employed.