An overview is given on the background of different turbulence models that can be used to compute boundary layers in external aerodynamics, such as for aircraft. The overview includes algebraic models (Cebeci-Smith, Baldwin-Lomax), a half-equation model (Johnson-King), two-equation models ( K-E-K-), and a differential Reynolds-stress model. The models were compared for boundary layer without and with streamwise pressure gradient. The models were also used to study the large-Reynolds number scalings (wall function and defect layer). The comparisson of models and the sealing analysis are described in three seperate journal contributions.