Abstract: This Article examines the unrecognized origins and scope of the judicial presumption of police expertise: the notion that trained, experienced officers develop insight into crime sufficiently rarefied and reliable to justify deference from courts. That presumption has been widely criticized in Fourth Amendment analysis. Yet the Fourth Amendment is in fact part of a much broader constellation of deference, one that begins outside criminal procedure and continues past it. Drawing on judicial opinions, appellate records, trial transcripts, police periodicals, and other archival materials, this Article argues that courts in the mid-twentieth century invoked police expertise to expand police authority in multiple areas of the law. They certified policemen as expert witnesses on criminal habits; they deferred to police insights in evaluating arrests and authorizing investigatory stops; and they even credited police knowledge in upholding criminal laws challenged for vagueness, offering the officer’s trained judgment as a check against the risk of arbitrary enforcement. Complicating traditional accounts of judicial deference as a largely instrumental phenomenon, this Article argues that courts in the midcentury in fact came to reappraise police work as producing rare and reliable “expert” knowledge. And it identifies at least one explanation for that shift in the folds and interconnections between the courts’ many diverse encounters with the police in these years. From trials to suppression hearings to professional activities outside the courtroom, judges experienced multiple sites of unique exposure to the rhetoric and evidence of the police’s expert claims. These encounters primed judges to embrace police expertise not only through their deliberative content, but also their many structural biases toward police knowledge. This development poses important and troubling consequences for the criminal justice system, deepening critiques of police judgment in criminal procedure and raising novel concerns about the limits of judicial reasoning about police practices.