If you are arrested, the police may not search your cell phone unless (1) you give them permission to do so or (2) they obtain a search warrant. In Riley v. California, the United States Supreme Court held that absent exigent circumstances (i.e. a public safety emergency or something similar), the police may not search the data on your cell phone without a warrant. In it's ruling, SCOTUS noted how most Americans who own cell phones keep on their record a digital record of nearly every aspect of their lives. With modern technology, we carry our photo albums, medical information, an address book, trade secrets and other sensitive information in our pockets and purses. As such, our substantial expectation of privacy with regards to phone data outweighs law enforcement's desire for investigative convenience.