"Zero Sum" is the twenty-first episode of the fourth season of the American science fiction television series The X-Files. It premiered on the Fox network on April 27, 1997. It was directed by Kim Manners, and written by Frank Spotnitz and Howard Gordon. "Zero Sum" included appearances by William B. Davis, Laurie Holden and Morris Panych. The episode helped to explore the overarching mythology, or fictional history of The X-Files.