Protecting the highly personal location data stored on or generated by digital devices is one of the 21st century’s most important privacy issues. In 2017, the Supreme Court finally took on the question of how law enforcement can get ahold of this sensitive information.

Whenever you use a cell phone, whether to make calls, send or receive texts, or browse the Internet, your phone automatically generates “cell site location information” (CSLI) through its interactions with cell towers. This means that cell providers like AT&T, Verizon, and T-Mobile have records of everywhere your phone has been, going back months and even years. And since almost everyone has a cell phone, cell providers have these records for nearly everyone.

The government has long argued that it doesn’t need a warrant to obtain CSLI from cell providers because of two 1970’s Supreme Court cases, Smith v. Maryland and United States v. Miller. Smith and Miller are the basis for the Third Party Doctrine, which holds that information you voluntarily share with a “third party”—such as deposit and withdrawal information shared with banks (Miller) or numbers dialed on a phone shared with the phone company (Smith)—isn’t protected by the Fourth Amendment because you can’t expect that third party to keep the information secret.

For years, courts around the country have been deeply divided on whether the Third Party Doctrine should apply to CSLI or whether the invasiveness of long term monitoring it enables should require a more privacy-protective rule. EFF has been involved in almost all of the significant past cases on this issue.

In June, the Supreme Court agreed to consider that question in Carpenter v. United States. In Carpenter, the government obtained 127 days of the defendant’s cell phone records from MetroPCS—without a warrant—to try to place him at the locations of several armed robberies around Detroit. As in other cases, the government argues that Mr. Carpenter had no reasonable expectation of privacy in these records, which it claimed were simultaneously incriminating but not precise enough to reveal his exact location and movements over those 127 days.

EFF filed briefs both encouraging the court to take the case and urging it to reject the Third Party Doctrine. We noted that cell phone usage has exploded in the last 30 years, and with it, the technologies to locate users have gotten ever more precise.

We attended the Supreme Court oral argument in Carpenter in late November. While it is always risky to predict the outcome of a case based on the argument, it appears that a number of the justices are concerned about the scope and invasiveness of tracking individuals using CSLI. Justice Alito agreed that this new technology is raising serious privacy concerns; Justice Roberts recognized that never before has the government had the ability to track every individual; and Justice Sotomayor was concerned that your cell phone could be tracked into the most intimate places like your bedroom or your doctor’s office.

The Supreme Court’s opinion in Carpenter will have important ramifications for the future, especially as our phones generate more—and more precise—location information every year, which is shared with third parties. But its reach could extend far beyond cell phones. Other increasingly popular technologies will force courts to consider these issues as well. For example, “Internet of Things” devices like smart thermostats that track when we’re home and when we’re not, watches that record our heart rates and rhythms, and clothing that tracks our emotions and communicates directly with retail stores may constantly generate and share data about us with little to no volition on our part.

The Supreme Court’s opinion in Carpenter will come out next year. We hope it meets this trend of sophisticated tracking with strong Fourth Amendment protection.

This article is part of our Year In Review series. Read other articles about the fight for digital rights in 2017.