"Crisis on Earth-X" is the fourth Arrowverse crossover event, featuring episodes of the live-action television series Supergirl, Arrow, The Flash, and Legends of Tomorrow on The CW. The crossover began on November 27, 2017, with Supergirl and Arrow, and concluded on November 28, with The Flash and Legends of Tomorrow. In "Crisis on Earth-X", Barry Allen and Iris West's friends come to Central City for their wedding, only for the proceedings to be interrupted by interlopers from the analogous universe of Earth-X, where the Axis powers claimed victory in World War II.

The Arrowverse is an American superhero media franchise and shared universe that is centered on various interconnected television series based on DC Comics superhero characters, primarily airing on The CW as well as web series on CW Seed. The series were developed by Greg Berlanti, Marc Guggenheim, Andrew Kreisberg, Geoff Johns, Ali Adler, Phil Klemmer, Salim Akil, and Caroline Dries. Set in a shared fictional multiverse much like the DC Universe and DC Multiverse in comic books, it was established by crossing over common plot elements, settings, cast and characters that span six live-action television series and two animated series.