Ad hoc coalitions (AHCs) are an indispensable but scantly conceptualized part of global governance. In recent years, several typologies and classifications of global governance arrangements have been provided, mostly differentiating them based on their organizational design features of degree of formality and membership composition. These do not capture AHCs and the role they play in global governance. In this article, we not only provide a conceptualization of AHCs, but also propose ways in which AHCs fit within the broader global governance architecture. We argue that what sets AHCs apart is not so much their (in)formality or membership, but rather their short-notice creation, their task-specific purpose and their temporarily circumscribed existence. We therefore define AHCs as autonomous arrangements with a task-specific mandate established at short notice for a limited time frame. We then develop a research agenda on the nature and future of AHCs, including their short- and long-term relationship with other multilateral arrangements in the global governance architecture. This is important, as we do yet not know how AHCs complement, compete and impact on international organizations and international crisis response.