Some say that the mass collection of U.S. phone records is a gross invasion of privacy. Others say that it is necessary to keep us safe. But what does the U.S. Constitution say? "The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.” Is collection of phone records a “search” or “seizure"? If so, is it “unreasonable”? Does it require a particularized warrant and probable cause? These are among the most consequential—and controversial—constitutional questions of our time.