We know from the corporate social responsibility (CSR) literature that civil society organizations (CSOs) play a crucial role in helping businesses transition towards more responsible and sustainable business models. Yet, in the circular economy (CE) field, such understanding is still scarce. This article develops a theoretical framework for understanding the dynamic interplay between business and civil society in CE transitions, understanding their mechanisms and strategies for cooperation and contestation. We develop the concept of 'CE boundary work' to understand how CSOs interact with firms to bring about CE innovation, outlining three ideal types: campaign based, resource efficiency based and circular design based. This contributes to our understanding of how CE transitions can be brought about effectively through cross-sectoral interplays between civil society and business.