The high costs for the development, erecting und operation of wind turbines are connected to very high expectations for a reliable and low-maintenance operation and require a precise knowledge of the loads and stresses to be expected. The transfer of knowledge from smaller wind turbines and possibly other concepts succeeds only to a limited extent. Rather comprehensive simulation approaches to determine wind loads, operational conditions and possible resonances are already used since many years. By means of simulation models, the natural frequencies can be determined and compared to possible excitations. The simulation of the operation of the wind turbine under different wind speeds allows the calculation of component loads as a basis for the further design process. The paper concentrates on the possibility of using multibody-system simulation models in the design process of gearboxes for wind turbines and the associated dynamic properties of the complete system by the example of the 15 MW reference wind turbine of the National Renewable Energy Laboratory (NREL). The comprehensive factors which influence the load distribution in the gearing of the first planetary gear stage require a detailed consideration of the elasticity of all relevant components. Based on the developed gearbox design and a detailed multibody-system simulation model, the influence of the level of detail of the model on the resulting natural frequencies and the occurring load distribution in the gearing of the planetary gear stages can be discussed. The present results show that findings on the required level of detail of simulation models cannot be applied to new turbines independently of the power class.