Both our common law constitution and human rights law treat liberty as a central value. Yet, immigration detention remains less constrained, both normatively and institutionally, than other forms of detention. International human rights bodies and courts, and indeed domestic courts, routinely review and indeed sometimes condemn detention of migrants. Yet, that jurisprudence has been subject to a convincing critique, for failing to properly scrutinise the necessity of immigration detention. Many scholars have thus pointed out the law's anomalously indulgent approach to immigration detention, compared with other forms of deprivation of liberty. Yet, powerful as this critique is, it sometimes fails to address prior questions concerning the political purposes and legal grounds of detention. By examining these grounds and purposes, both legitimate and illegitimate, the lecture will aim to elucidate the manner in which immigration law produces reasons to detain, and thwarts any test of necessity from effectively constraining the state's power to detain migrants. The diverse approaches of the UN Human Rights Committee, European Court of Human Rights and Court of Justice of the European Union will be contrasted. The likely impact of new EU norms on detention of asylum-seekers and pre-removal detention will also be explored.