Detention as part of migration control is sometimes portrayed as a ‘necessary adjunct’ of the state’s power to control immigration. This characterization is a masking device, obscuring the grounds of detention (or the lack thereof) from proper scrutiny. It has convincingly been argued that human rights law fails to scrutinize the necessity of immigration detention. Many scholars have pointed out the anomalous approach to assessing the legal justifications for immigration detention, compared with other forms of deprivation of liberty, which are more powerfully constrained by human rights law. Yet, cogent as this critique is, it sometimes fails to interrogate the related questions concerning the legal grounds of detention. A ground is a particular form of legal reason, which both explains and justifies the official action in question. By examining the question of grounds, this article aims to elucidate the manner in which immigration law itself produces reasons to detain, and by doing so creates detainable subjects, migrants. Basic liberty-protective principles and practices developed in other areas of law are notably absent. This state of affairs is not inevitable, and legal alternatives are within reach.