The Supreme Court, in recent decisions, has indicated that the purpose of the fourth amendment exclusionary rule is to deter police officers from engaging in illegal searches and seizures. However, the rule can serve as a deterrent only to the extent that officers understand the rule and know what acts are prohibited. In this study, the authors draw on research conducted in four northeastern police departments. An examination of officers' knowledge of the laws revealed a significant lack of understanding about what the rules prohibit. As a result, even if all officers were to obey the law, there would be substantial deviation because of police mistakes about what it requires. The study showed that even extensively trained officers are mistaken about one-quarter of the time about the lawfulness of an action. Further, the study revealed that officers would deliberately carry out what they believed to be an illegal intrusion in at least one of the scenarios described in the questionnaire. The authors recommend officer training in search and seizure rules and direct sanctions against officers to promote adherence to these rules. 144 notes, 13 tables