From Mexico to the Bahamas, Mauritania to Lebanon, Turkey to Saudi Arabia, South Africa to Indonesia, Malaysia to Thailand, immigration-related detention has become an established policy apparatus that counts on dedicated facilities and burgeoning institutional bureaucracies. Until relatively recently, however, detention appears to have been largely an ad hoc tool, employed mainly by wealthy states in exigent circumstances. This paper uses concepts from diffusion theory to detail the history of key policy events in several important immigration destination countries that led to the spreading of detention practices during the last 30 years and assesses some of the motives that appear to have encouraged this phenomenon. The paper also endeavors to place the United States at the center of this story because its policy decisions appear to have played an important role in encouraging the process of policy innovation, imitation, and imposition that has helped give rise to today’s global immigration detention phenomenon. Nevertheless, many US offshore practices have not received nearly the same attention as those of other important destination countries. More broadly, in telling this story, this paper seeks to flesh out some of the larger policy implications of the externalization of immigration control regimes. Just as offshore interdiction and detention schemes raise important questions about custody, accountability, and sovereignty, they should also spur questions over where responsibility for the wellbeing of migrants begins and ends.