On Tuesday, the Supreme Court will confront the profound impact of new location-tracking technologies on Americans' privacy. The case, U.S. v. Jones, presents the question of whether law enforcement needs a warrant before planting a GPS tracking device on a person's car. The answer to this question is important in its own right, but the case is likely to have broader implications.

Attaching a GPS to a car isn't the only way the government can track people's movements. In fact, everyone with a cell phone is already carrying a device that the government can use to track his or her location. As a result, the principle at stake in this case may well shape our privacy rights in the years and decades to come.

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