Since 1988, Sweden and Finland have collaborated bilaterally on thermal energy storage with respect to information exchange and collaborative R&D projects. The two countries have both investigated underground thermal energy storage for nearly two decades, and have similar bedrock—the Fenno-Scandian granitic rocks. This paper reviews the work performed in the field of combined rock cavern and borehole heat stores, concerned with construction technology, costs and design principles. One example is an asymmetric store, in the form of 40- to 60-m-long horizontal boreholes between two rock caverns, with the caverns themselves comprising only about 10% of the total storage volume. This design has a specific cost of $US0.40 million/ GWh and $US0.24 million/ GWh for storage capacities of 6 GWh and 36 GWh, respectively. Half of the total construction cost relates to the rock cavern part of the store.