A unanimous Supreme Court on Wednesday endorsed a major advance for personal privacy with far-reaching and welcome implications for restricting government access to private information.

Reviewing two related cases, eight justices supported Chief Justice John Roberts' opinion that the police must obtain a warrant from a judge before accessing the content of a suspect's cell phone. The ninth justice, Samuel Alito, wrote a separate concurring opinion.

The court held that a cell phone typically contains information that reveals more in combination than any single record, often in great detail covering years of an individual's life, and consequently falls under the Fourth Amendment's protection of "persons, houses, papers, and effects" against arbitrary searches.

That view asserts the so-called "mosaic theory," holding that bits of information that might reasonably be available from public sources can be put together in a way that invades privacy rights protected by the Fourth Amendment's prohibition of "unreasonable searches and seizures."

Taken together with a recent appeals court ruling that cell-phone location information is personal information, even though recorded by cell-phone service providers, the Supreme Court ruling on cell phone content serves as an important curb against an intrusive government. Together, they provide an up-to-date response to the 1979 Smith case that opened the door to mass collection of American telephone call records by the National Security Agency.

In the sweeping Smith case, the Supreme Court ruled that police could subpoena call records collected by a telephone company without a court order because the telephone subscriber could not have a reasonable expectation that the records were private. That ruling left a vast area of modern life, such as bank and credit card records, library loans, even scanned grocery purchases, subject to government search without a court warrant. The Patriot Act, which further enhanced police and intelligence agency powers, relies on the Smith ruling.

In contrast, the new requirement that police obtain a warrant before searching the contents of a cell phone will limit law enforcement and to some degree handicap its investigations. That point was acknowledged by Chief Justice Roberts, who correctly emphasized that "privacy comes at a cost." He suggested that exceptions could arise in the future where police might be justified in searching cell phones, but left that to future courts to decide.

Justice Alito agreed that new technology "calls for a new balancing of law enforcement and privacy interests." But he called on Congress and the state legislatures to define that balance in laws that give clear instruction to police, thereby relieving the courts of the burden of effectively writing new privacy laws.

Whether the new balance is struck by the courts or by Congress, it is clearly required, as demonstrated by the NSA's overreach into the private lives of Americans.

Wednesday's Supreme Court decision recognizes the right of individuals to personal privacy and the need to restrain government access to technology that contains an ever increasing amount of personal information.