In November 2014, Berlanti expressed interest in
Supergirl existing in the
Arrowverse, the same universe as his other series
Arrow and
The Flash, and in January 2015,
The CW president Mark Pedowitz revealed that he was also open to a crossover between the series and networks (due to Berlanti executive producing all three and The CW being co-owned by CBS). However, CBS Entertainment chair Nina Tassler stated that month that "those two shows are on a different network. So I think we'll keep
Supergirl to ourselves for a while." In August 2015, Tassler revealed that while there were no plans at the time to do crossover storylines, the three series would have crossover promotions. Pedowitz regretted passing on the series when presented it in mid-2014, saying, "We hadn't launched
The Flash yet, we weren't ready to take on another DC property. In hindsight we probably should've gone that direction...Sometimes you lose great shows." In January 2016, during the
Television Critics Association press tour, he said that The CW was still interested in a crossover with
Supergirl if the producers could find a way to do it, and Berlanti added that while no official conversations had taken place, internal ones had concerning how a crossover would work. He also noted that for a crossover to happen during
Supergirl first season, it would have to be figured out "in the next month or so". Glenn Geller, Tassler's successor at CBS, then stated on the matter, "I have to be really careful what I say here. Watch and wait and see what happens." On February 3, 2016, it was announced that
Grant Gustin, who appears as
Barry Allen / Flash on
The Flash, would appear in the eighteenth episode of the first season, "
Worlds Finest". While no plot details on the episodes were released at the time, Ross A. Lincoln of
Deadline Hollywood noted that "the in-universe reason" for the crossover was due to Barry's ability to travel to various dimensions, thus implying that
Supergirl exists on an alternate Earth to
Arrow and
The Flash in a multiverse.
The Flash episode "
Welcome to Earth-2" confirmed this, showing an image of Benoist as Supergirl during a sequence where characters travel through that multiverse. The earth that the series inhabits is Earth-38 in the Arrowverse multiverse, and has been informally referred to as "Earth-CBS" by
Marc Guggenheim, one of the creators of
Arrow. During the second season, Supergirl appears in "
Invasion!", a crossover episode of
The Flash,
Arrow and
Legends of Tomorrow, when she's recruited by Barry Allen and Cisco Ramon at the end of "
Medusa" to help fight off an invasion by the
Dominators.
Supergirl and
The Flash also featured in a musical crossover, featuring several covers of existing songs along with two original numbers. After "Invasion!", Guggenheim felt "If there's an appetite for it from the fans and from the network," the crossover next year could be "a proper four-part crossover." At the 2017
Paleyfest event, Kreisberg reiterated the creative team's intention to do a full four-way crossover the following year. At
San Diego Comic Con in 2017, it was confirmed that another four-way crossover would take place, with Supergirl playing a larger role than the previous season. The four-way crossover event, titled "
Crisis on Earth-X", took place on November 27 and 28, 2017, across
Supergirl and
Arrow (on the first night) and
The Flash and
Legends of Tomorrow (on the second night). In May 2018,
Arrow star
Stephen Amell announced at The CW
upfronts that the next
Arrowverse crossover would feature
Batwoman and
Gotham City. The crossover, titled "
Elseworlds", aired in December 2018, ahead of a potential 2019 solo series for the character.
Supergirl was confirmed to have a participating episode in August, which closed out the three-part crossover, trading nights with
The Flash just for the event. Therefore, the show's participating episode aired on Tuesday, December 11. The end of "Elseworlds" teased the next crossover event, "
Crisis on Infinite Earths".
Supergirl's episode opened the five-part crossover on December 8, 2019, with the final two installments airing on January 14, 2020. At the end of the event, the new Earth-Prime was formed, which saw Earth-38 merged with the former Earth-1 and
Black Lightnings earth, creating a fictional universe where all of the CW series exist together.
Standalone spin-off In October 2019, The CW and Warner Bros. Television announced development on a spin-off series titled
Superman & Lois, with
Tyler Hoechlin and
Elizabeth Tulloch reprising their roles as Clark Kent / Superman and Lois Lane. In January 2020,
Superman & Lois was ordered to series. The series premiered on February 23, 2021. Originally presented as being set in the same continuity as
Supergirl, the series was retroactively established to be set in an adjacent universe to the Arrowverse in its second season finale. ==Other media==