Nowadays, one of the biggest challenges to face in the energy sector is the transition to more sustainable methods of production and consumption of energy. In order to achieve this goal big changes, mainly focused on energy saving, are needed. Collective Self-Consumption and Energy Communities are going to play a key role in this transition, enhancing the efficiency of consumption and bringing benefits to both citizens and environment. Thanks to this new kind of organization, there is going to be a switch from the current electricity system, mainly centralized and based on fossil fuels, to a decentralized system, mostly powered by green and non-polluting energy.