Research on Open Innovation has increasingly emphasised the role of communities in creating, shaping and disseminating innovations. However, the comparability of many studies has been hampered by the lack of a precise definition of the community construct, and the research on Open Innovation has to date not been well connected to insights from research on the role of transformational leaders and the networking of champions and promotors across organisational boundaries. For this reason, this paper introduces a new construct of ‘innovation communities’ based on promotor theory, which it defines as ‘networks of promotors’. It proposes a comprehensive concept of the quality of interaction in innovation communities, and presents findings of three case studies, which explore the role of promotors and networks of promotors in Open Innovation. The case studies reveal that such transformational leaders as promotors, and especially their close and informal co-operation across functional and organisational boundaries, play a key role in Open Innovation.