The Supreme Court has held that the sniff of a trained drug detection dog is not a “search” under the Fourth Amendment because the dog does no more than reveal the existence of “contraband.” As technology advances, courts will have to confront new forms of purported “contraband-only” investigative techniques, ranging from “hash” searches for child pornography and electronic sniffers to “gun detectors” and swipes of counterfeit credit cards. This Note provides a three-step framework to evaluate those technologies under existing doctrine and addresses a question that has so far received little attention: What is “contraband” for purposes of the Fourth Amendment? This Note proposes using a definition of “anticipatory contraband” that requires not only examining whether something is illegal to possess as a matter of substantive criminal law but also demonstrating that the police knew in advance that there was no lawful right to private possession under the circumstances. This Note then applies the three-step framework and the theory of anticipatory contraband to a variety of new technologies. This analysis reveals that while the contraband-only doctrine encompasses a variety of investigative techniques, the “in-advance” requirement is surprisingly restrictive, requiring police and courts to be alert to the possibility of compromising legitimate private information.