This article introduces a special issue on the expanding research agenda on institutional fragmentation. The term refers to the growing diversity and challenges to coordination among private and public norms, treaties, and organizations that address a given issue area of international politics. International relations scholars increasingly address this phenomenon, framing it with alternative concepts like regime complexes or polycentricity. A considerable part of the existing debate remains focused on whether a centralized or polycentric governance architecture is preferable. Instead, as this special issue shows, domains of global environmental governance—like climate change, biological diversity, renewable energy, and forestry—are already fragmented. It is time to address new, more pertinent questions and help advance institutionalist research on this phenomenon. We introduce four major research themes for analyzing the fragmentation of different domains of global environmental governance: taking stock, causes, consequences, and responses.