Energy communities are gaining increasing relevance in the transition toward more decentralized, decarbonized and digitalized energy systems. They represent a heterogeneous phenomenon, a characteristic that explains the difficulty in providing a definition of what they are. This chapter offers some conceptual clarity by building a comprehensive taxonomy of energy communities that practitioners can rely on. Five different categories of communities are distinguished based on their main function in the energy system and the level of maturity of their business models. This chapter also highlights the strengths and weaknesses of the different categories and the policy and regulatory issues they raise.