This report summarizes experiences and lessons learned on Underground Thermal Energy Storage (UTES) systems from the participating EU project partners and is supplemented with input from publications on other relevant cases in, and outside, Europe. The report covers important experiences from the first pre-investigation phase and feasibility studies and throughout the construction phase, system integration and operations. Furthermore, different national legislative, political and public issues are described. The HEATSTORE project and this report comprise four different UTES systems: - High Temperature Aquifer Thermal Energy Storage (HT-ATES) - Borehole Thermal Energy Storage (BTES) - Pit Thermal Energy Storage (PTES) - Mine Thermal Energy Storage (MTES) The ideas behind MTES is state of the art and the HEATSTORE demonstration site in Bochum, Germany, will be the first of its kind. Experiences and lessons learned regarding this specific system is therefore not existing, see chapter 5. In Appendix I and II, specific report contributions from partners are included. These contributions have served as an important basis for this deliverable.