There are large knowledge gaps around the governance of the low carbon energy system transition in a smooth and participative way, ensuring that citizens are at the centre of the required fundamental transformation and enabling the full efflorescence of their creative potential. Social innovation is a prime way to tap into that potential while Collective Action Initiatives (CAIs), a social innovation in itself, are a prime way to mobilize people and to ensure the acceptance for and participation in the necessary transition process. However, both social innovation and CAIs lack proper scientific and field-tested understanding of their development and factors for success. As of today, the role of citizen-driven CAIs (e.g. energy communities, cooperatives, purchasing groups) and their contribution to the energy transition has neither been quantified at an aggregate level, nor has their contribution potential been estimated or understood in sufficient depth. The COMETS project aims to fill these knowledge gaps by quantifying the European-wide aggregate contribution of CAIs to the energy transition at national and European levels by investigating their evolution and scaling up at an in-depth level in six selected countries. The main expected impacts of the project are two-fold. Firstly, COMETS will advance the scientific knowledge on the motives, desires, objectives and barriers of such collective action initiatives and their historical and future role in the energy transition. Building on the information gathered and tested for its robustness, we will then co-develop and test supportive tools together with CAI members, decision makers and the scientific community. Lastly, these stakeholders will then be able to exploit the main outputs of COMETS, namely a Supporting Platform for CAIs, the enhanced knowledge base, scenarios and roadmaps for spreading CAI models, even after the project is concluded.