In this editorial introduction, we provide a context for our special issue by outlining the major approaches and issues related to immigration detention in liberal, democratic states. We are concerned that there is no commonly accepted definition of detention, and so we endeavour to provide one here. Beyond the need for greater conceptual clarity, we suggest that a pressing question is why immigration detention is continuously expanding despite mounting evidence that the practice harms people, does not deter irregular immigrants, and fails to ensure more efficient and effective immigration and asylum determination systems. We introduce the major themes from each contribution and explain how they both address this question and initiate new ones. Collectively, these papers demonstrate that immigration detention is embedded in, and essential for, wider immigration and penal apparatuses; yet, it also operates by its own logics, which in turn shape population boundaries in new ways both within and beyond the sovereign state. We conclude that immigration detention warrants special critical attention and a more robust research agenda. Copyright © 2012 John Wiley & Sons, Ltd.