District heating and cooling systems move heat in urban areas. Heat and cold are generated in central supply units by heat or cold recycling, renewables, or by direct heat or cold generation. The heat and cold demands should be concentrated in order to keep low distribution costs. District heating and cooling systems substitute ordinary primary energy supply for heating and cooling. Therefore, district heating and cooling increase both energy efficiency and decarbonisation in the global energy system. However, district heating and cooling is a highly underestimated energy efficiency and decarbonisation method in contemporary energy policy, both nationally and internationally.