The research center of Zero Emissions Buildings (ZEB) has a goal of eliminating the greenhouse gas emissions associated with all phases of building development and use. This is achieved through more sustainable building construction and more efficient energy use. The Norwegian government has a similar goal of achieving zero energy buildings as a standard by 2020. This has led to proper investigation in technological solutions that can help to achieve these goals. In a net-ZEB perspective, combined heat and power (CHP) is considered as a potential energy supply solution for buildings. CHP is seen as an emerging technology which has the potential to reduce primary energy consumption and the associated greenhouse gas emissions. This is achieved through concurrent production of electricity and heat using the same fuel. However, since the thermal output of CHP is substantially larger than the electrical output, the potential offered by CHP systems depend on their suitable integration with the thermal demand of the building. In this thesis, a simulation model is used to investigate the performance of a CHP system compared to a conventional gas boiler system in a multi-family building that complies with the Norwegian building norm, TEK10. Different operational strategies are applied to the CHP model to investigate its optimal integration in domestic dwellings. Analyzing the simulation results indicates that the CHP system gives primary energy savings in all operational strategies, but operating the system in follow thermal mode represents the greatest savings. Applying load management resulted in further savings, and the fuel efficiency did increase, achieving a value of 75.1% on a higher heating value (HHV) basis. The CHP device is more capable of covering the electricity demand as peaks are shaved. This implies that CHP is better suited for buildings with stable electricity and heat demand. Electric demand following operation did however result in poorer primary energy savings and the corresponding CHP efficiency did decrease due to poorer heat recovery efficiency and frequent part load operation. Using renewable upgraded biogas as fuel in thermal following mode did result in the highest primary energy savings. Primary energy consumption was reduced by 34.3%, and the corresponding system efficiency based on primary energy was 70.7% on a HHV basis. From an environmental perspective, it has been found that the CHP system is more favorable when the CO2-emission factor for electricity is high. This is due to the reduction in electricity imports from the grid, and the part substituted electricity covered by the electricity exports from the CHP system. The greatest reduction in grid imports was seen when the CHP-device was set to follow the electrical demand of the building without restriction in thermal surplus. The CHP was able to cover 88.27% of the electricity demand, but the system efficiency decreased as significant amounts of heat was wasted due to overproduction. The highest amount of exports was seen when load management was implemented in thermal demand following mode, and represented 76.61% of the produced electricity. Using the current CO2-emission factor for the UCPTE electricity mix, a reduction in CO2 emissions was seen for all CHP configurations. The use of renewable fuel resulted in the greatest savings, and emissions were reduced by 71.91% compared to the gas boiler, representing a tremendous reduction. The use of natural gas as fuel resulted in significantly lower savings. The best case achieved a 26.58% reduction compared to the reference system. When using the net-ZEB definition, only CHP fuelled on renewable fuel did achieve CO2-savings. This questions the environmental viability of today’s CHP systems as the CO2-emission factor for electricity is expected to decrease over the coming years due to an expected increase in use of renewable fuels. Further research should therefore be done in order to enable an efficient CHP technology based on renewable fuels. This will decrease the emissions significantly, making CHP more competitive.