City-regions as sites of sustainability transitions have remained under-explored so far. With our comparative analysis of five diverse European city-regions, we offer new insights on contemporary sustainability transitions at the urban level. In a similar vein, the pre-development and the take-off phase of sustainability transitions have been studied in depth while the acceleration phase remains a research gap. We address this research gap by exploring how transitions can move beyond the seeding of alternative experiments and the activation of civil society initiatives. This raises the question of what commonalities and differences can be found between urban sustainability transitions. In our explorative study, we employ a newly developed framework of the acceleration mechanisms of sustainability transitions. We offer new insights on the multi-phase model of sustainability transitions. Our findings illustrate that there are no clear demarcations between the phases of transitions. From the perspective of city-regions, we rather found dynamics of acceleration, deceleration, and stagnation to unfold in parallel. We observed several transitions-transitions towards both sustainability and un-sustainability-to co-evolve. This suggests that the politics of persistence-the inertia and path dependencies of un-sustainability-should be considered in the study of urban sustainability transitions.2 of 25 innovations vary from organic urban food production, to the upcycling of household products, reusing waste as a resource, climate neutral transport, product sharing and sufficiency-oriented lifestyles. These initiatives start to create new social networks and synergies, develop new narratives on the future of cities and become relevant actors in urban governance arenas [1,2]. These are dynamics that seem to differ from the earlier phases of sustainability transitions, which are focused on nurturing innovations in protected spaces. This raises the question if and how urban societies already move beyond the seeding of alternative experiments and the activation of community initiatives for sustainability action. It is, therefore, important to examine local agents of change such as community initiatives and learn how sustainability in practice is further diffused and integrated into the life of cities [3,4]. So far, there is a limited understanding of the processes through which the innovations introduced by such transition initiatives (henceforth TIs) are taken up or transferred beyond the community that created, facilitated, and nurtured them.Our research addresses this theoretical gap and offers a new perspective on the understanding of the context-led and agency-led conditions that enable or hinder the acceleration of sustainability transition in city-regions. We do so based on in-depth empirical case studies and a cross-case comparative analysis of five European city-regions. In comparing the acceleration of sustainability transitions in five city-regions, we explore the following research questions: