Circular business models (CBMs) have gained much attention in the fashion retail industry as a solution and opportunity for transitioning to more sustainable practices. Resale (i.e., second-hand) has proven to have an especially high growth potential in fashion retail. However, for CBMs to play an important role in the transition to a more sustainable fashion retail industry, they must be able to scale and displace linear consumption. What this means is currently obscure. Consequently, the purpose of this thesis is to theoretically and empirically explore and describe the scaling of fashion retail CBMs. This is addressed through qualitative research detailed in five appended papers. Using different theoretical perspectives and methods, such as literature reviews, interview studies, and a case study, the thesis and its sub-studies examine how the scaling of CBMs can be described, as well as which strategies are used in the fashion industry. Based on the interplay between theoretical frameworks, from both general management literature and social innovation literature, and empirical material, the thesis suggests a synthesis of three general perspectives on how the scaling of fashion retail CBMs can be described: firstly, as a phenomenon that mainly concerns the focal organisation; secondly, as a phenomenon that concerns both the focal organisation and its impact on the external environment; and finally, as a process that is formed and shaped over time. Additionally, seven main strategies employed by fashion retail CBMs in their efforts to scale were also identified. The findings also show that, depending on the perspective, CBMs can have different objectives when it comes to scaling. If scaling is perceived as mainly concerning the focal organisation, then strategies, for example, will focus on internal improvements for organisational efficiency. If it is perceived using a wider perspective, also including impacts on the external environment, then strategies that focus on changing formal and informal institutions will also be employed. The process perspective views these strategies as they connect over time in order to achieve scale. Understanding that scaling can be described in different ways and have different strategic foci is an important contribution to both theory and practice. The different perspectives influence how both managers and researchers approach the issue of CBM scaling, and what challenges and strategies they include in their analysis. By articulating and systematically describing different perspectives on CBM scaling, this thesis brings clarity to a complex concept.