"Synchrony" is the nineteenth episode of the fourth season of the American science fiction television series The X-Files. It was written by Howard Gordon and David Greenwalt and directed by James Charleston. The episode aired in the United States on April 13, 1997 on the Fox network. The episode is a "Monster-of-the-Week" story, a stand-alone plot which is unconnected to the series' wider mythology. "Synchrony" earned a Nielsen rating of 11.3, being watched by 18.09 million people upon its initial broadcast. The episode received mixed to positive reviews from television critics.

The fourth season of the American science fiction television series The X-Files commenced airing on the Fox network in the United States on October 4, 1996, concluding on the same channel on May 18, 1997, and contained 24 episodes. Following the filming and airing of the season, production began on The X-Files feature film, which was released in 1998 following the show's fifth season.