Institutionalism gives priority to structure over agency. Yet institutions have never developed and operated without the intervention of interested groups. This paper develops a conceptual framework for the role of agency in historical institutionalism. Based on recent contributions following the coalitional turn and drawing on insights from sociological institutionalism, it argues that agency plays a key role in the creation and maintenance of social coalitions that stabilize but also challenge institutions. Without such agency, no coalition can be created, maintained, or changed. Similarly, without a supporting coalition, no contested institution can survive. Yet, due to collective action problems, such coalitional work is challenging. This coalitional perspective offers a robust role for agency in historical institutionalism, but it also explains why institutions remain stable despite agency. In addition, this paper forwards several portable propositions that allow for the identification of who is likely to develop agency and what these actors do.