When it comes to new technologies, it’s easy to imagine the worst. In the 1960s, people feared transistors would spell the end of privacy, with mini electronics listening to our private conversations. Today, there’s alarm about facial recognition. But just as transistors didn’t give rise to widespread eavesdropping, neither will facial recognition lead to pervasive surveillance. Writing in the Wall Street Journal, Daniel Castro explains that the most common facial-recognition applications are benign—and Fourth Amendment protections, coupled with deeply held views about civil liberties, put limits on what government can do.