As wind turbines continue to increase in size, so do the loads acting on the drivetrain.To handle the higher loads the mass of the drivetrain needs to increase. A better understandingof this mass increase can result in lighter drivetrain design and providean insight which drivetrain design has the lowest mass potential for large rotor diameters.In this thesis the limiting mechanisms, mass and torque density of threewind turbine drivetrains are investigated for an increasing rotor diameter. The limitingmechanisms in this thesis provide a theoretical limit to the minimum requiredmass of a drivetrain for increased loading. Through an extensive literature study thelimiting mechanisms are found and three scaling models are developed to calculatethe torque density of the drivetrains. The drivetrains that are being considered in thisthesis are the drivetrain with gearbox and high-speed generator, the direct drivetrainand the hydraulic drivetrain. The following research questions are answered in thisthesis:• What are the limiting mechanisms of the three investigated drivetrain types?• What are the achievable torque densities of the three investigated drivetrainconfigurations for increasing rotor diameters?Two limiting mechanisms are found for the drivetrain with gearbox and high-speedgenerator: Tooth flank stress (1.72 GPa) and Root bending stress (0.24 GPa). TheTooth flank stress is found to be governed by the Hertzian contact strength while theroot bending stress is governed by the flexural strength. Two limiting mechanismsfor the RFPMSG are found for the airgap flux density (1 T) and the current loading(30 - 200 kA/m) in the generator windings. The airgap flux density is limited by thesaturation of the stator teeth material in the generator whereas the current loadingis limited by the heat dissipation of the windings and stator laminations. For thehydraulic pump, the limiting mechanism is found at the interaction of the camringand the cam roller. The maximum stress that is allowed at this point is determinedby the Hertzian contact strength of the material and is found to be (1.72 GPa). Usingthe limiting mechanisms of the three drivetrain configurations, scaling models forthe mass and torque density of the three drivetrains have been developed.