A down hole motor assembly (11) comprising a hydraulic drive portion (12), a drill bit (13) operatively connected to the drive portion (12) and a stator (14) connected to the drill string. The drive portion (12) is arranged to be rotated relative to the stator by the supply of fluid to a motor which drives the drive portion (12). The assembly (11) also includes a ratchet mechanism (19) operatively connected between the stator (14) and the drive portion (12), the ratchet mechanism being arranged to permit the drive portion (12) to rotate relative to the stator (14) in a normal forward drilling sense and to prevent relative rotation between the drive portion (12) and the stator (14) in the opposite sense.
