A magnetic drive system comprising three members, a first and second of which form an input member and an output member each arranged to rotate relative to the third member, wherein two of the members have respective sets of permanent magnets, the two sets having different numbers of magnetic poles, and the other of the members comprises a cylindrical body having a plurality of pole pieces embedded therein, the pole pieces being arranged to modulate the magnetic field acting between the magnets, and wherein one of the pole pieces is of varying radial thickness.