A controller for use with a working machine comprising a machine body and a load handling apparatus coupled to the machine body and moveable by a lift actuator with respect to the machine body, wherein the controller is configured to receive: a signal representative of a lateral inclination angle of the machine body with respect to a lateral reference orientation; and a signal representative of a stability of the working machine. The controller is further configured to determine a permissible movement range of the load handling apparatus with respect to the machine body or a longitudinal reference orientation and issue a signal for use by an element of the working machine including the lift actuator, which in response to the signal issued by the controller is configured to restrict or prevent movement of the load handling apparatus outside of the permissible movement range relative to the machine body or longitudinal reference orientation, the permissible movement range being dependent on the signal representative of a lateral inclination angle of the machine body with respect to a lateral reference orientation and the signal representative of the stability of the machine. The permissible movement range is obtained by determining a stability envelope for the working machine and a location of the working machine's centre of gravity. The permissible movement range is chosen such that the working machine's centre of gravity remains in the stability envelope across the whole of the permissible movement range.