This Article uses economic concepts to understand search and seizure law, the law governing government investigations that is most often associated with the Fourth Amendment. It explains search and seizure law as a way to increase the efficiency of law enforcement by accounting for external costs of investigations. The police often discount negative externalities caused by their work. Search and seizure law responds by prohibiting investigative steps when external costs are excessive and not likely to be justified by corresponding public benefits. The result channels government resources into welfare-enhancing investigative paths instead of welfare-reducing steps that would occur absent legal regulation. This perspective on search and seizure law is descriptively helpful, it provides a useful analytical language to describe the role of different Fourth Amendment doctrines, and it facilitates fresh normative insights about recurring debates in Fourth Amendment law.