As of Wednesday, all smartphones sold in California must be equipped with a so-called kill switch.
The law is intended to make it pointless to steal a smartphone. The kill switch lets owners remotely lock their phone if it's stolen so no one can use it.
Officials say the technology has already reduced crime.
Police reported 3.1 million smartphone robberies in 2013, but only 2.1 million last year.
CA law requires kill switch in all smartphones sold