At the heart of transition research lies the question of how to “scale up” sustainable alternatives from a protected niche to the creation of mainstream practices.While upscaling processes are often seen as an essential element that contributes to societal transformation, upscaling itself remains a fuzzy concept.We argue that some fundamental dilemmas of upscaling can be identified, for example, the different understanding of the concept by researchers and practitioners. The dilemmas should be addressed in a more reflexive way by those from the worlds of science and practice who are involved in collaborative research settings.