Every geothermal bore will have its own individual characteristic relating mass flow and pressure, and no two bores will behave in exactly the same manner. In a ‘wet’ field a third variable, the dryness fraction, is added to the pressure and the mass flow as a component element of the bore characteristic. Where several bores simultaneously supply geothermal fluids to a utilisation plant, whether for power generation or other purpose, it is necessary to devise a system of control whereby the bores, despite their different characteristics and allowing for possible changes in these characteristics, will collectively provide the required quantities of fluids at the required pressures and with a minimum of waste, and whereby any unwanted fluids are automatically disposed of. In a complex system of transmission and plant, there may be points at which the prime necessity is to supply a certain quantity of steam and where pressure must be permitted to vary in such a manner as to ensure this supply. At other points the prime necessity may be constancy of pressure and the quantity of steam delivered to the plant must be permitted to vary in such a manner as to ensure this constancy. The transmission of hot water, collected at saturation pressure, must be effected without permitting the water to boil, as this could give rise to unpredictable and possibly disastrous water hammer phenomena. To ensure this, an adequate system of controlling the rate of hot water flow is necessary, taking due account of pipe friction, gradient and accelerations (both positive and negative). In addition to these requirements for controlling the flow of geothermal fluids, both steam and hot water, in such a manner that the utilisation plant may function smoothly and correctly, precautions must also be taken to control these fluids so as to prevent damage to plant and injury to personnel as a result of the bursting of pipes or pressure vessels on account of excessive pressures or their collapse under vacuum conditions, or as a result of water hammer arising from entrained water in the steam mains. All these requirements call for adequate automatic controls and safeguards, which are described in the paper. There will also be a need for conditions at certain points in the plant and in the field to be monitored at a central control point, to enable the plant operators to observe what is happening from afar and to take corrective action in emergencies by means of remote control.