Apparatus for controlling a cryogenic cooling system is described. A supply gas line and a return gas line are provided which are coupled to a compressor and to a mechanical refrigerator via a coupling element. The coupling element is in gaseous communication with the supply and return gas lines and supplies gas to the mechanical refrigerator. The pressure of the supplied gas is modulated by the coupling element in a cyclical manner. A pressure sensing apparatus monitors the pressure in at least one of the supply and return gas lines. A control system is used to modulate the frequency of the cyclical gas pressure supplied by the coupling element in accordance with the pressure monitored by the pressure sensing apparatus. An associated method of controlling such a system is also described.