This volume presents the proceedings of ECSCW'09, the 11th European Conference on Computer Supported Cooperative Work. Each conference offers an occasion to critically review our research field, which has been multidisciplinary and committed to high scientific standards, both theoretical and methodological, from its beginning. These proceedings represent discussions and contributions to ongoing challenges. One challenge comes from emerging new technologies connected to 'social computing', gaming, as well as applications supporting citizen participation in their communities. As boundaries...