The invention relates to a drive system, in particular a downhole drilling assembly (10). In the preferred embodiment, a bearing mechanism (14,24) of the assembly (10) is isolated from a gear mechanism (20) of the assembly (10), to prevent failure of the bearing mechanism (14, 24) due to vibration and heat generated by the gear mechanism (20) in use. A lower bearing unit (24) of the bearing mechanism (24) is coupled to the gear mechanism (20) by a shock eliminating coupling assembly (22), which prevents the transmission of shock loads to the gear mechanism (20). Also, the gear mechanism (20) is coupled through a shock eliminating coupling assembly (18) to a turbine (16) and thus to an upper bearing unit (14). Sealing assemblies (60, 116) are provided for the gear mechanism (20) and the upper bearing unit (14) to prevent drilling fluid ingress and consequent damage. The assembly (10) may also carry a torsionally flexible drive shaft (46).