Judging from the number of communities and cities striving or claiming to be sustainable and how often eco-development is invoked as the means for urban regeneration, it appears that sustainable and eco-development have become “the leading paradigm within urban development” (Whitehead 2003). But what is it that is driving these urban transformations? Clearly, there are many probable answers to this complex question and in what follows we will focus on one particular catalyst of change – urban design competitions. Considered as field changing events (Lampel and Meyer 2008, Anand and Jones 2008), urban design competitions are understudied mechanisms for bringing about field level changes. Drawing on actor network theory, this paper examines how urban design competitions may bring about changes within the professional field through the use of intermediaries such as a sustainable planning/assessment tool. The context for our study is urban regeneration in one Danish city, which had been suffering from industrial decline and which is currently investing in establishing a “sustainable city”. Based on this case study we explore how the insights and inspiration evoked in working with the tool influenced the design and planning practices.