Transmissions are an essential part of a vehicle powertrain. An optimally designed powertrain can result in energy savings, reduced environmental impact and increased comfort and reliability. Along with other components of the powertrain, efficiency is also a major concern in the design of transmissions. The churning power losses associated with the motion of gears through the oil represent a significant portion of the total power losses in a transmission and therefore need to be estimated. A lack of reliable empirical models for the prediction of these losses has led to the emergence of CFD (Computational Fluid Dynamics) as a means to (i) predict these losses and (ii) promote a deeper understanding of the physical phenomena responsible for theselosses in order to improve existing models. The commercial CFD solver STAR-CCM+ is used to investigate the oil distribution and the churning power losses inside two gearbox configurations namely an FZG (Technical Institute for the Study of Gears and Drive Mechanisms) gearbox and a planetary gearbox. A comparison of two motion handling techniques in STARCCM+ namely MRF (Moving Reference Frame) and RBM (Rigid Body Motion) models is made in terms of the accuracy of results and the computational requirements using the FZG gearbox. A sensitivity analysis on how the size of gap between the meshing gear teeth affects the flow and the computational requirements is also done using the FZG gearbox. Different modelling alternatives are investigated for the planetary gearbox and the best choices have been determined. The numerical simulations are solved in an unsteady framework where the VOF (Volume Of Fluid) multiphase model is used to track the interface between the immiscible phases. The overset meshing technique has been used to reconfigure the mesh at each time step. The results from the CFD simulations are presented and discussed in terms of the modelling choices made and their effect on the accuracy of the results. The MRF method is a cheaper alternative compared to the RBM model however, the former model does not accurately simulate the transient start-up and instead provides just a regime solution of the unsteady problem. As expected, the accuracy of the results suffers from having a large gap between the meshing gear teeth. The use of compressible ideal gas model for the air phase with a pressure boundary condition gives the optimum performance for the planetary gearbox. The outcomes can be used toeffectively study transmission flows using CFD and thereby improve the design of future transmissions for improved efficiency.