The European Union has set an ambitious goal of a total or at least near-total decarbonisation until 2050. Buildings represent the single largest contributor of greenhouse gases and are the largest single energy consumer across the continent. As more and more people move into densely populated urban areas, the sustainable supply of energy for space heating, cooling and the production of domestic hot water will become an ever-increasing issue. District heating and cooling systems present themselves as an ideal solution in this process as they enable the production and utilization of renewable and waste energy at locations where they are available as well as their distribution to the locations where they are need at high energy densities. They also allow for increased flexibility in terms of production technologies and energy sources as the majority of the system and the associated costs are linked to the distribution side which consists of hot water pipes and systems of distributing the water. Finally the utilization of district heating and cooling systems in a combination with power to heat and heat storage technologies can lock the potential to increase the overall energy systems flexibility and in turn, increase its capacity to utilize intermittent renewable electricity sources. The main objective of this doctoral thesis is the assessment and quantification of the impact of district heating and district heating and cooling on the potential for the utilization of intermittent renewables in moderate and Mediterranean climates. The hypothesis of the thesis is that a high share of district heating and district heating and cooling in a combination with power to heat and heat storage can have a significant and positive impact on the potential of the energy system as a whole to integrate more intermittent renewables into its electricity generation without generating excessive critical excess of electricity in Europe’s continental and Mediterranean climates. The doctoral thesis is based on six papers, four of which have been published in high impact factor CC journals, one in a Q2 journal and one conference paper. The presented results have confirmed the proposed hypothesis. As seen in the results of the thesis, systems with a high share of district heating and cooling could easily absorb 4.000 MW of wind and 4.000 MW of PV which would produce upwards of 17,13 TWh of electricity, or roughly 73% of the total electricity demand (including the additional demand from heat generation through heat pumps and electric boilers), with roughly 5% critical excess of electricity production. The results also demonstrate that district cooling, even though it represents a much smaller demand then district heating and is integrated into the scenarios with much smaller shares, still decreases critical excess of electricity production by up to an additional 12% when comparing scenarios with and without cooling. Demand side management and flexibility of energy systems is an increasingly important element as the share of intermittent renewable energy sources such as wind and solar power increase. District heating and cooling can enable this flexibility while also providing an adaptable and sustainable source of heating and cooling energy for Europe’s densely populated cities. It is a key technology which needs wider adoption in Europe’s mild and Mediterranean climates where it can bring all the benefits it is already providing to other territories across the continent.