The phrase "the criminal justice system " is ubiquitous in discussions of criminal law, policy, and punishment in the United States-so ubiquitous that, at least in colloquial use, almost no one thinks to question the phrase. However, this way of describing and thinking about police, courts, jails, and prisons, as a holistic "system, " became pervasive only in the 1960s. This essay contextualizes the idea of "the criminal justice system" within the longer history of systems theories more generally, drawing on recent scholarship in intellectual history and the history of science. The essay then recounts how that longer history converged, in 1967, with the career of a young engineer working for President Johnson's Crime Commission, whose contributions to the influential report The Challenge of Crime in a Free Society launched the modern and now commonplace idea of "the criminal justice system." Throughout, the essay reflects upon the assumptions and premises that go along with thinking about any complex phenomenon as a "system" and asks whether, in the age of mass incarceration, it is perhaps time to discard the idea, or at least to reflect more carefully upon its uses and limitations.