The new long term strategy of the European Union is to become climate neutral and powered by mostly renewable energy by 2050. The transition to a near or fully renewable energy system is a very complex process that relies on distributed energy production, the integration of electricity, thermal, and gas grids through energy conversion and storage technologies, as well as demand side flexibility and energy saving. Regional and municipal governments have a key role in governing the transition because they are the smallest political units in most countries, they have a considerable degree of autonomy in their jurisdiction, and they are close to the actors who actually have to implement the changes.Currently, regions and municipalities are failing to live up to the task and would need to roughly double their ambitions. However, they are facing both technical, political and administrative challenges when it comes to designing and implementing strategic energy plans. The technical challenge is: how to choose which specific energy sources and technologies to rely on in which sectors?. The political challenge is: how to increase the acceptability and legitimacy of the transition among politicians, businesses and local communities?. The administrative challenge is: how to coordinate all the changes in the different subsectors and how to mobilize the necessary resources for that? Co-creation is an emerging governance strategy that occurs when government agencies, businesses, non-governmental organizations, and unorganized communities work together to define common problems and design and implement innovative solutions. The author proposes that cocreation could provide a useful alternative to market-or government-led strategies for governing the renewable energy transition. The aim of this thesis is to study whether the challenges that regions and municipalities face could be overcome by the co-design and co-implementation of strategic energy plans. As such, it is one of the first studies that applies the proliferating knowledge about co-creation in political science to the energy sector.So far, co-creation in the energy sector has been insufficiently conceptualized; existing empirical studies have focused mainly on the co-creation of energy plans but not their implementation; there are no good examples of the institutionalization of co-creation in new governance bodies; and the feasibility of co-creation outside Western Europe is largely unknown. This thesis contributes to filling these knowledge gaps by conducting a literature review and analyzing three empirical cases: the co-design of a regional energy plan 4 in Ida-Virumaa in Estonia, and the co-implementation of municipal energy plans in Ringkøbing-Skjern and Sønderborg in Denmark.The results show that co-creation as a governance strategy for renewable energy transitions can be divided into: 1) collaborative process, 2) institutional design and leadership, 3) outputs and outcomes and 4) antecedent conditions. The thesis offers new contributions in all of these categor...