This study explores how challengers theorize change in mature fields and how their theorization evolves in view of contestation from the incumbents. As peripheral actors, challengers often lack the legitimacy and resources to enact major change, and their attempts to change the field’s arrangements do not go uncontested by the incumbents. We studied how a social enterprise configured solutions to improve access to vaccination services in resource-poor countries and, in doing so, changed the institutional arrangements in the mature field of global health. Based on a longitudinal qualitative study, we extend the concept of theorization in institutional theory. In particular, we show how incumbents’ contestations lead the challengers to complexify their theorization by elaborating solutions to secure endorsement and introduce change in the global health field. We highlight how, in pluralistic mature fields, challengers may develop multiple streams of change aimed at diverse categories of incumbents in response to different issues, interests, and institutional arrangements, yet simultaneously form a master theorization by assembling the diverse solutions together.
