Abstract: The constitutionality of a search or seizure typically depends upon the connection between the target of that search or seizure and some allegation of illegal behavior — a connection assessed by asking whether the search or seizure is supported by probable cause. And yet, central as probable cause is to the Fourth Amendment’s administration, no one seems to know what it means, or how it operates. Indeed, the Supreme Court has gone so far as to insist that it is “not possible” to define the term, holding instead that the probable cause inquiry entails no more than the application of “common-sense” to “the totality of the circumstances.” That doctrinal approach is routinely criticized as an “I know it when I see it” mode of jurisprudence that is ill equipped to safeguard civil liberties in the face of competing and weighty law enforcement demands. Viewed charitably, however the Supreme Court’s refusal to elaborate on the meaning of probable cause stems from an understandable desire for doctrinal flexibility in the face of widely varying law enforcement-civilian interactions. This tension between doctrinal flexibility and structure is the animating dilemma of probable-cause jurisprudence — a dilemma that this Article attempts to navigate, and ultimately to resolve. To do so, it urges a rejection of an often invoked (if not always followed) tenet of Supreme Court doctrine: probable cause unitarianism. That dominant idea, expressly endorsed in many of the Court’s leading precedents, holds that whatever probable cause means, it ought to entail the same basic analytic method, judged by the same substantive standard, from one case to another. On close inspection, however, the Supreme Court does not always practice what it preaches. Rather, beneath the surface of its probable cause canon there are seeds of an alternative — and superior — conception of probable cause, which this Article terms probable cause pluralism. On this view, “probable cause” is an open-textured and capacious idea that can comfortably encompass multiple distinct analytic frameworks and multiple different substantive standards, each of which can be tailored to the unique epistemological and normative challenges posed by different types of Fourth Amendment events. Probable cause, as the case may be, can be statistically driven or intuitively assessed; it can demand compelling evidence of illegal behavior or only an occasionally satisfied profile; it can presume the credibility of some types of witnesses, while treating others with deserved skepticism or disbelief. It can, in short, come to mean something, so long as it gives up on meaning any one thing in all cases. Because probable cause’s pluralism is both nascent and implicit, it is also undertheorized, having escaped sustained or comprehensive analysis by either the Court or its commentators. As a result, probable cause pluralism, in its current form, is at best a stunted and haphazard collection of disparate and disconnected ideas. This Article’s central contribution is to bring those ideas together, refining and synthesizing them into a comprehensive account of what a pluralist theory of probable cause could and should like in practice. Specifically, by organizing probable cause around three central analytic axes — which in turn ask how to assess evidentiary claims, how to assess proponents of such claims, and how to determine the certainty thresholds for those two assessments — the Article constructs a universally applicable framework for determining the constitutionality of any given search or seizure. With that framework in hand, scholars and jurists will be better equipped to reason through all the many and varied cases to come, and better able to assess all the many cases that have come before.