Criminal law and immigration law were once entirely separate fields of governance, but over the last few decades the boundary between the two fields has grown less and less distinct. Immigration crimes now account for a majority of all federal prosecutions; deportation is widely seen as a key tool of crime control; immigration authorities run the nation’s largest prison system; and state and local law enforcement officers work hand in hand with federal immigration officials. This article traces these trends and assesses their significance. The rise of an intertwined regime of “crimmigration” law has generally been attributed to some combination of nativism, overcriminalization, and a cultural obsession with security, but it also exemplifies, and has helped to reinforce, a crucial and underappreciated development in legal culture – a rising tendency to treat legal rules and legal procedures as interchangeable tools, to be brought to bear pragmatically and instrumentally on an ad hoc basis. Ad hoc instrumentalism of this kind has genuine strengths, but it also raises significant concerns about the rule of law and political accountability. The accountability concerns, in particular, are exacerbated by two other features of our newly merged system of immigration enforcement and criminal justice: its bureaucratic opacity and its selective application.