In a general mindset of ‘local elaboration’ of sustainable development, cities are logical loci for action: they do not only concentrate (future) consumption and production  and are hence at the origin of unsustainability ‘symptoms’-, they simultaneously are the operational units in which concrete actions can be envisaged, designed, (politically) facilitated and effectively rolled out. Whenever cities engage in this innovative, ambitious and responsible task of change for integrated sustainability, an undoubtedly major amount of learning emerges; and vice versa, sound knowledge/best practices on how to proceed with local sustainability oriented change processes could be a firm support for local actors in their quest for effective and efficient action. In this paper, we present ‘Urban Transition Labs’ (UTL) as settings in which real life trajectories of sustainable development in cities are deployed and at the same time carefully observed; in a co-creative collaboration between actors and researchers (transdisciplinary research).