Later this year, the Supreme Court will decide if police can track a person’s cell phone location without a warrant. It’s the most important privacy case in a generation.

For all of the attention paid to former FBI Director Jim Comey’s highly anticipated testimony before the Senate intelligence committee last Thursday, the most important constitutional law development from last week took place across the street (and three days earlier), when the Supreme Court agreed to hear argument in Carpenter v. United States later this year—though exactly when, we’re not sure.

Carpenter raises a specific question about whether Americans have an expectation of privacy in historical “cell-site location information” (“CSLI”). The Petitioner, Timothy Carpenter, was one of two defendants convicted for his role in a series of armed robberies in Michigan and Ohio, based in part on 127 days of CSLI data that placed him between ½ and 2 miles from the robberies around the time they were committed.

On appeal, the Sixth Circuit affirmed his conviction, holding that Carpenter had no expectation of privacy in his CSLI data–and so the government did not need a warrant before obtaining it and admitting it at trial. Whether the Supreme Court endorses or rejects this logic, the answer has enormous implications for privacy rights—and the Fourth Amendment—more generally. Indeed, it’s no exaggeration to suggest that Carpenter will be the most important Fourth Amendment case that the Supreme Court has heard in a generation.

It’s no exaggeration to suggest that Carpenter will be the most important Fourth Amendment case that the Supreme Court has heard in a generation.

A quick refresher on the Fourth Amendment to the Constitution: that’s the one that protects individuals from “unreasonable searches and seizures,” and says warrants must be issued with “probable cause,” and specify a particular area or domain to be searched. As the Supreme Court has interpreted it, the Fourth Amendment applies to a government search—and the warrant requirement is usually triggered—whenever the search intrudes upon a “reasonable expectation of privacy.” Thus, whether we have such an expectation of privacy in particular types of property or information is typically the key to whether the Fourth Amendment applies to government actions against the same.

At the heart of the dispute in Carpenter is the “third-party doctrine,” the idea that we surrender our constitutional expectation of privacy whenever we voluntarily share non-content information with third-parties, such as our phone companies, internet service providers, financial institutions, and so on. The third-party doctrine brings together two distinct strands of Fourth Amendment theory—that individuals’ voluntary conduct can effect a form of “consent” to government searches, and that individuals cannot assert a privacy interest in property they no longer own or possess.

Without an expectation of privacy in such information, the government may collect it without having to satisfy the Fourth Amendment—even if, in some cases, it must nevertheless satisfy more modest statutory requirements that Congress has imposed. Thus, although Edward Snowden’s 2013 disclosure of the bulk telephone metadata collection prompted substantial public debate over the constitutionality of such surveillance, it is difficult to see the argument that it was unconstitutional, thanks to the third-party doctrine (the Second Circuit instead invalidated it on narrower, statutory grounds).

Although the phone records program is perhaps the most prominent example, there are hundreds of other government surveillance programs or protocols, the legitimacy of which is predicated on the third-party doctrine.

But as the Snowden disclosures also underscored, technology has dramatically changed the privacy implications of the third-party doctrine. As Justice Sonia Sotomayor explained in a 2012 concurrence,

This approach is ill suited to the digital age, in which people reveal a great deal of information about themselves to third parties in the course of carrying out mundane tasks. People disclose the phone numbers that they dial or text to their cellular providers; the URLs that they visit and the e-mail addresses with which they correspond to their Internet service providers; and the books, groceries, and medications they purchase to online retailers.