In-situ measurement of the thermal properties of soils and rocks can play an important part in developing cost-effective and sustainable designs for closed-loop geothermal and ground energy systems. A thermal response test (TRT) is used to determine in-situ thermal properties. This involves injecting a finite amount of heat energy into a closed-loop borehole over a period of several days. During a test the principal parameters measured are the incoming and outgoing temperatures of the heat transfer fluid; the flow rate of the fluid; and the heat energy input. Test data can be interpreted to provide the following parameters, all of which are necessary for efficient design of closed-loop systems: the initial ground temperature, the thermal properties of the ground and the equivalent borehole thermal resistance.