The modern HDR concept originated at the Los Alamos National Laboratory and was first demonstrated at Fenton Hill, New Mexico (USA). Experience gained during the development of the deeper HDR reservoir at Fenton Hill clearly showed that HDR reservoirs are formed by opening pre-existing, but sealed, multiply-connected joint sets. Subsequent flow testing indicated that sustained operation of HDR systems under steady-state conditions is feasible. The most significant remaining HDR issues are related to economics and locational flexibility. Additional field test sites are needed to advance the understanding of HDR technology so that the vast potential of this resource can be economically realized around the world.