If you are arrested, up until very recently, the police were allowed to search you and examine your possessions. They could look through your wallet. Rifle through your purse. And as cell phones evolved into smart phones, they applied this same logic to those devices, and felt at liberty to explore the data on your smart phone, too.
But on Wednesday, June 25th, 2014, the Supreme Court changed that. They ruled unanimously that police officers cannot search your smart phone or any other mobile device without a warrant. Even if they are arresting you.