The energy system of the building is integrated in a larger energy system. The focus in this thesis is on the interface between the building and the surrounding energy system. Of special interest is how changes in the building energy system or the surrounding system affect each other and how they can interact. Depending on the decision criteria applied by the designer of the building energy system, various strategies can be used vis-a-vis the surrounding energy system. These are discussed as a background to the case studies in the appended papers. The overall objective for most of the case studies is minimisation of cost. One of the papers also deals with an environmental objective. When the Nordic electricity prices are harmonised with the European market, which has a pronounced diurnal variation in spot prices rather than a significant seasonal variation, the energy systems in the Nordic countries have to be run accordingly. This is why short time steps are used throughout the year in some of the models in order to reflect the diurnal variations in the spot price for electricity. All case studies concern buildings within a district heating network, where heat is supplied with combined heat and power plants, and boilers. Some energy conservation measures are also analysed, such as extra wall insulation and new windows. Aspects studied include how life cycle cost is affected by changes in the surrounding energy system, and how the surrounding energy system is affected when the measures on the demand side are compared with investments on the supply side. A number of techniques, for example optimisation models, are developed to analyse these situations. Finally, a suggestion is made as to how the surrounding energy system and the building energy system can interact, when jointly acting on the Nordic electricity market.