The Supreme Court ruled Wednesday that police must obtain warrants before snooping through people's cellphones, delivering a unanimous decision that begins to update legal understanding of privacy rules to accommodate 21st-century technology.

Police agencies argued that searching through data on cellphones was no different from asking someone to turn out his pockets, but the justices rejected that, saying a cellphone holds the most personal and intimate details of someone's life and falls squarely within the Fourth Amendment's privacy protections.

"The fact that technology now allows an individual to carry such information in his hand does not make the information any less worthy of the protection for which the Founders fought," Chief Justice John G. Roberts Jr. wrote in the unanimous opinion. "Our answer to the question of what police must do before searching a cell phone seized incident to an arrest is accordingly simple — get a warrant."