Field measurements were made, for a period of one year, of the performance of five heat pump systems used for space heating and tap water heating in single-family houses. The purpose was to show the potential of heat pump technology in order to raise the acceptance of the technology, and to show that heat pumps are energy-efficient in cold climates. Another goal was to learn more about how SPF is influenced by different system designs. Parts of this study are the Swedish contribution to IEA HPP Annex 37 "Demonstration of field measurements of heat pump systems in buildings". The study covered two buildings with geothermal heating, one with geothermal heating combined with solar panels, one brine/water heat pump connected to ground storage combined with solar panels, and one building with an air/water heat pump combined with solar panels. The parameters monitored in the project were temperature and volume flow of heat carriers and tap water, and electrical energy used by compressors and circulation pumps. SPF, energy coverage ratio, energy savings and CO2 reduction were calculated. The results show that there is a potential for energy savings and CO2 reduction by using heat pump technology. Energy savings up to 75 % of the supplied heat have been demonstrated. CO2 reductions are influenced by which type of heating system the heat pump system is compared to, and how the electrical energy is produced. When reporting results from field measurements, it is important to state which electrical components that are included in the analyses. This study used the system boundaries defined in the European SEPEMO-build project. SPF for the five installations ranged from 2.8 to 4.0 when all electrical equipment was included in the calculations, which implies that heat pumps are energy-efficient in cold climates. The SPF value is not the only performance indicator to consider when evaluating heat pump systems. Also energy demand, energy coverage ratio, CO2 emissions, availability are most important parameters for heating systems in which a heat pump is combined with other heat sources, and for systems with large proportion of domestic hot water production.