The rising global energy demand and the large proportion of the buildings, but also rising expectations in terms of comfort call for measures in order to reduce the energy consumption in the building sector. Due to the long life cycle of buildings, especially the time of operation is very important next to the planning phase, in which the basis for an energy efficient building is placed. The first part of the work deals with the technical basis for the development of an energy monitoring system. Furthermore, the recommended system configuration and the state of the art in the field of measurement technology are described. For this part of the paper a literature search based on books, standards, conference proceedings, guidelines, laws and in scientific articles was realized. The second part presents the conception of an energy monitoring system for an existing office building in Vienna based on the acquired knowledge from the first chapter. Afterwards, the consultations to implement the different types of meters, as well as for integrating them into the existing building management system are described. In the third part the measurement data is processed and evaluated. Furthermore, the first optimization measures are verified in terms of their effectiveness. Through energy saving measures by the use of the building automation system and because of improving the hydronic conditions of the cold water system, the cooling energy consumption for the office building has been reduced. The electrical energy consumption remained almost constant, however, optimization measures in the field of the ventilation systems and the chiller could be developed. The heat consumption for the three investigated periods remained at a constant level. Optimization measures for the heating system were developed, but could not be implemented in the evaluation period. Finally, the results and findings of the thesis are discussed and the calculated key figures are compared with benchmark values.