Probable cause is generally cast in judicial opinions and the scholarly literature as a fixed probability of criminal activity. In the weeks before the September 11 attacks, FBI headquarters, applying such an unbending standard, rejected a warrant application to search Zacarias Moussaoui’s laptop computer. This article, which begins with an analysis of the Moussaoui episode, argues that the probable cause standard should be calibrated to the gravity of the investigated offense and the intrusiveness of a proposed search. Tracing the evolution of probable cause from the common law through its American development, the article argues that the Supreme Court’s current insistence on a “single standard” lacks historical support. Probable cause should be recast within a reasonableness framework, embracing the common sense view that not all searches equally trench on privacy concerns and not all crimes equally threaten the social order.