Police must obtain a warrant before installing GPS technology to track suspects, the U.S. Supreme Court ruled unanimously Monday.
Five justices said allowing police to use electronic surveillance without warrants would cause constitutional and privacy problems.
The ruling is an unusual setback for law enforcement, which is increasingly using the technology to track suspects.
"The Supreme Court's decision is an important one because it sends a message that technological advances cannot outpace the American Constitution," Prof. Donald Tibbs, at Earle Mack School of Law at Drexel University, said.
"The people will retain certain rights even when technology changes how the police are able to conduct their investigations," he explained.
The justices made it clear they would continue to look into the use of high-tech surveillance.