This chapter focuses on the case of extraterritorial military detention by the US and the UK—two countries that quickly deployed and then repeatedly refined their detention policies during the nearly two decades following the terrorist attacks of September 11, 2001. Military detention is arguably one of the quintessential national security functions where deference to executive discretion is strongest. As such, it is an activity that differs markedly from the types of practices that form the core work of many domestic administrative agencies, and administrative law scholarship tends to ignore the national security domain. Yet even here, in a realm seemingly so insulated from administrative law norms, agencies in both the US and the UK have implemented a variety of administrative rules and procedures, as well as non-judicial administrative tribunals to assess the status of detainees. Although the US and the UK followed different pathways, both countries have ultimately come to embrace administrative law frameworks for military detention. And both countries have gradually moved to protect, at least to a limited extent, the core administrative law values of rationality, transparency, participation, and procedural protection even as they have rejected fully judicialized detention processes. This comparative case study therefore illustrates the significance of administrative law values in the area of national security and points toward the need for further scholarship at the intersection of national security law and administrative law.