In Major Privacy Win, Supreme Court Rules Police Need Warrant To Track Your Cellphone

He stressed, however, that this is a narrowly focused opinion that leaves intact other precedents when it comes to dealing with financial information, banking and office records.

Roberts noted that the decision also allows for warrantless cell-tower location information searches in emergencies and for national security purposes.

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At oral arguments in November, the justices seemed torn about whether to break with the so-called third party doctrine. Adopted decades ago, that doctrine says that there is no reasonable expectation of privacy when an individual shares information with a third party -- for example, the phone company, which knows what telephone numbers the individual calls and receives. Therefore, police do not have to get a search warrant to gain access to those numbers.

But in recent years, the justices have expressed discomfort with that rule of law as applied to the modern digital age, when cellphones carried in a person's pocket can track locations day and night, and when email and text addresses tell a huge amount about an individual's contacts and lifestyle.