After years of legal debate, the Supreme Court has told the cops to keep their hands off Americans' cell phones–at least until they get a search warrant.

The court released a landmark decision Wednesday morning in the case of Riley vs. California, forbidding warrantless police searches of the contents of arrestees' cell phones. The ruling opinion notes that cell phones have in fact become tiny computers in Americans' pockets teeming with highly private data, and that gaining access to them is now fundamentally different from rifling through someone's pockets or purse. "A decade ago officers might have occasionally stumbled across a highly personal item such as a diary, but today many of the more than 90% of American adults who own cell phones keep on their person a digital record of nearly every aspect of their lives," the opinion reads.

That ruling contradicts the argument from U.S. prosecutors that a search of a cell phone should instead be treated "as materially indistinguishable" from a search of any other box or bag found on an arrestee's body. "That is like saying a ride on horseback is materially indistinguishable from a flight to the moon," the Supreme Court's ruling reads. "Modern cell phones, as a category, implicate privacy concerns far beyond those implicated by a cigarette pack, a wallet or a purse."

Two cases brought the issue of warrantless cell phone searches to the Supreme Court's attention. In one, a California man was charged with assault and attempted murder in relation to a street gang in which he was allegedly a member. Cops searched his smartphone without a warrant and found videos and photos that prosecutors argued linked him to the "Bloods" gang. In the second case, a Boston man had his cell phone searched when he was arrested after an apparent drug sale. By finding his home address on his flip phone, police were able search his home and find a larger stash of drugs. Both defendants argued that the warrantless searches violated the fourth amendment.

Wednesday's ruling sides with both defendants and declares the searches in their cases unconstitutional.

Privacy groups celebrated the ruling; Hanni Fakhoury of the Electronic Frontier Foundation calls it a "bright line, uniform, pro-privacy standard," and the American Civil Liberties declared the decision "revolutionary."

"By recognizing that the digital revolution has transformed our expectations of privacy, today’s decision...will help to protect the privacy rights of all Americans," reads a statement from Steven R. Shapiro, the ACLU's national legal director. "We have entered a new world but, as the court today recognized, our old values still apply and limit the government’s ability to rummage through the intimate details of our private lives."

In its ruling, the court also rejected prosecutors' notion that the ability to remotely wipe or lock a phone required police to search the devices immediately upon arrest before evidence could be destroyed–an ironic argument given that police around the country have pressed for those same remote "kill switch" abilities. The court responded that cops can easily prevent evidence from being remotely destroyed by turning the phone off or removing its battery, or putting it into a Faraday cage that blocks radio waves until a search warrant can be obtained.

But perhaps the most remarkable portion of the ruling is its recognition of the unique nature of modern smartphones as personal objects. Even calling them mere "cell phones" is a misnomer, it says. "The term 'cell phone' is itself misleading shorthand," the ruling reads. "Many of these devices are in fact minicomputers that also have the capacity to be used as a telephone. They could just as easily be called cameras, video players, rolodexes, calendars, tape recorders, libraries, diaries, albums, televisions, maps or newspapers."

The EFF's Fakhoury points to language in the ruling about how the sheer volume of data stored on cell phones makes them fundamentally different from other personal objects that might contain private information: The opinion specifically refers to the 16 gigabytes of storage in the top-selling iPhone model, containing "millions of pages of text, thousands of pictures, or hundreds of videos." This statement that the quantity of information searched matters—rather than merely the kind of information—might influence other privacy cases, Fakhoury says, such as those surrounding other police surveillance tactics or even those of the NSA.

"The court recognizes that two pictures reveal something limited but a thousand reveals something very different," says Fakhoury. "Does it mean something different when you’re collecting one person's phone calls versus collecting everyone’s phone calls over five years? Technology allows the government to see things in quantities they couldn’t see otherwise."

Read the full ruling below:

RILEY v. CALIFORNIA by Emily Dreyfuss