Both series share a showrunner in Kreisberg and both hail from Berlanti Productions, which also producesThe Flash's sister series Arrow and Legends of Tomorrow. While The Flash and Arrow have crossed over with each other several times, in addition to welcoming several Legends characters ahead of it's January debut, this marks the first crossover with Supergirl. Unlike the other superhero series, Supergirl is the only one to air on CBS.
Kreisberg shot down rumors of a spinoff as recently as December. "First of all, we don’t even know if we could, legally," he told THR at the time. "And quite frankly, just making the Arrow-verse, which is what we call Legends, Flash and Arrow work and then making Supergirl work, it's enough of a challenge without having to add a fourth player."
Although the shows are technically on rival networks, CBS Corp. is a part owner of The CW.
These superhero-themed crossovers have proven a ratings boon for The CW ever since The Flash debuted in 2014, joining veteran Arrow.
The Flash is the latest comic book character to visit Supergirl. Others include Martian the Manhunter, Non, the Toyman and the upcoming introduction of villain Silver Banshee. The show is also set to introduce a teenage Clark Kent later this season.
Supergirl airs Mondays at 8 p.m. on CBS. The Flash airs Tuesdays at 8 p.m. on The CW.
