Energy-efficient district heat was selected as the heating solution for the capital city as early as the late 1940’s, when a far-reaching decision on producing electricity and heat from the same fuel (CHP) and district heating was made in Helsinki. The Helsinki districts started utilising district heat at a rapid pace. This solution clearly improved the air quality in the city when building-specific chimneys became unnecessary. In terms of the climate, the decision was sustainable. The fuel efficiency of CHP in Helsinki is among the best in the world, over 90%, when in separate production it can reach just over 40% even in the best conditions. Helsinki also produces district cooling in the same processes with heat and electricity. The principle of 90 + 90 + 90 is realised in today’s Helsinki: over 90% of the fuel is utilised in energy generation, over 90% of the heated housing stock in Helsinki is connected to district heating, and the share of cogeneration of the produced district heat is over 90%.