The climate crisis is one of the greatest challenges of our time. To keep global warming below 2°C, worldwide greenhouse gas (GHG) emissions must be almost completely eliminated in the next two to three decades. In the EU, about 77% of total GHG emissions are caused by the utilisation of fossil fuels. In principle, GHG reduction can be achieved through behavioural change, energy efficiency measures or the replacement of fossil energy sources with renewable energy sources. Austria’s current government program aims to fully decarbonisation by 2040. However, there is no comprehensive strategy to achieve this goal. Nevertheless, there are various studies, initiatives and publications available that addresses different aspects of the decarbonisation of Austria. This thesis also deals with the decarbonisation of Austria but focuses on two aspects that have not been covered before, neither nationally nor internationally. The first aspect is about the determination of the optimal technology mix throughout the entire energy system to maximise exergy efficiency. The second aspect is the investigation of the national renewable gas import demand. To address these two aspects, the entire Austrian energy system, including all economic sectors, useful energy categories and the entire conversion chain from resource to energy service is considered. First of all, the status quo of the Austrian energy system was determined. This also involved the current demand for energy services. Then, an optimisation model was developed to identify the optimal technology mix for covering the total demand for energy services. In the last step, two scenarios were analysed for 2030, 2040 and 2050 to determine the total demand for renewable gases and the national potential of green hydrogen. This also includes a cost analysis. The results show that currently about 370 TWh/a of exergy are required to cover about 133 TWh/a of useful exergy to fulfil all energy services. This corresponds to an exergy efficiency of 36%. Optimal technology mix throughout the entire energy system would increase the efficiency to 58%. The future decarbonised energy systems will be mainly based on electricity and renewable gases. According to the scenarios, while almost the entire electricity demand can be covered nationally, at least 41% (40 TWh/a) of the renewable gas demand has to be imported in 2050. In the same year, in the business-as-usual scenario, the import share would be close to 100% (between 210 and 250 TWh/a). The implementation of efficiency and sufficiency measures reduces the import demand for renewable gases. Nationally produced green hydrogen has comparable costs to imported green hydrogen.