The booklet first presents the case of United States v. Wall, decided by the U.S. District Court of the Michigan Eastern District. The question at issue was whether or not an anonymous tip can justify a "Terry" stop or a warrantless search. The issue involves the fourth amendment, which recognizes the right of the people to be secure from unreasonable searches and seizures and provides that no warrants shall issue but upon probable cause. A warrantless search is generally considered unreasonable. The U.S. Supreme Court has adopted a "totality of the circumstances" analysis for probable cause. Under this standard, probable cause for a warrant to issue may be based on information from an anonymous informant that can be independently corroborated. The personal observations of the officers may also confer probable cause. In determining whether probable cause exists, courts more readily accept the judgment of a law enforcement officer if it is backed by a warrant issued by a magistrate. As a general rule, probable cause is required for searches, whether or not police obtain a warrant. A lesser standard, that of reasonable suspicion of criminal activity, may justify a minimal level of government intrusion under the fourth amendment when an important government interest is at stake. This booklet includes a posttest that covers the legal material presented, and a legal commentary deals with the specific issues raised by the "Wall" case.