For decades, the Supreme Court has maintained a rhetorical commitment to an ad hoc, case-by-case, totality of the circumstances approach to evaluating probable cause and reasonable suspicion. Despite the Court’s insistence that there are so many factors involved in the assessment of Fourth Amendment probabilities that one case will seldom be useful precedent for future cases, it has with increasing frequency endorsed the use of presumptive rules in this context. Such presumptions provide an appealing balance between the often competing imperatives of providing clear guidance to lower courts and law enforcement officers, on the one hand, and retaining the flexibility to ensure results consistent with Fourth Amendment principles in idiosyncratic cases, on the other. Additionally, although courts might evaluate the suitability of some such presumptions using quantitative analysis, in other cases, the use of qualitative, intuition-based judgment is necessary.