Energy communities—groups of citizens, social entrepreneurs and public authorities who jointly invest in producing, selling and managing enewable energy are expected to play a prominent role in the energy transition. Energy communities are fragile individually and they need to pool resources and coordinate their actions to become robust collectively. This paper adopts an ecosystem perspective and aims to identify characteristics that an energy community ecosystem should exhibit to help energy communities emerge, grow and eventually fully realise their potential to transform the energy sector. It compares energy communities in two countries, France and the Netherlands, where energy community ecosystems have attained uneven levels of maturity. We argue that an energy community ecosystem can fully realize its potential if: 1) it revolves around keystone actors that can foster diversity; 2) it is structured around local capacity builders that can act as catalysers; and 3) it develops both competing and symbiotic relations with incumbent energy actors.