Immigration detention is characterized by a tension between the prerogatives of sovereignty and the rights of non-citizens. While states have broad discretion over who is allowed to enter and reside within their borders, their decision to detain and deport is constrained by a number of widely accepted norms and principles. One of these is the principle of proportionality, which provides that any decision to deprive a person of his or her liberty must be proportionate to specific ends established in law. This paper argues that the proportionality principle, despite its close association to individual legal cases, can be used as a lens through which to assess the operations of detention centres, as well as overall detention regimes. In particular, the paper focuses on the intimate association between immigration detention and criminal incarceration as well as the institutional framework of detention estates, both of which raise a number questions about whether detention practices are proportionate to the administrative aims of immigration policy.