The guideline describes the appropriate design of thermal facilities for using the underground according to state of the art technology. It defines and explains the principles relating to heat flow in the underground and lists the approvals required under water and mining legislation. It deals with environmentally-compliant choice of materials, and describes the appropriate execution of boreholes. Futhermore, it explains how such facilities can be installed and integrated into systems. Specific applications are discussed: Heat pump systems that use groundwater or the underground with ground heat collectors and borehole heat exchangers, or energy storage facilities. The associated primary energy requirements and CO2 emissions are indicated. Consideration is given to potential environmental impacts, for example those resulting from leaks and from thermal and hydraulic effects.