The much anticipated superhero crossover episode between Arrow, The Flash, Supergirl and Legends of TomorrowFacebook/Arrow

This week, television is witnessing the biggest crossover episodes with four The CW shows collaborating to save the world. The week-long crossover sees lead characters from Supergirl, Arrow, The Flash and DC's Legends of Tomorrow, coming together to fight against aliens, the Dominators. Action heroes like Flash, Green Arrow, Felicity Smoak, White Canary, Thea Queen and Heat Wave are teaming up with Supergirl for the rendition.

While this is one of the most iconic moments on TV, there have been many crossovers in the past that have garnered viewers' attention and shot TRP off the roof. Here is a list of a few memorable crossover episodes that many still recall:

- The Vampire Diaries and The Originals:

The worlds of Mystic Falls and New Orleans came together earlier this year as the makers of The Vampire Diaries and The Originals aired a two-episode crossover event. The viewers were introduced to a secret society, Tyler Lockwood returns and was seen undergoing a werewolf transition and Enzo finds out about his family. But the highlight of the show was the relationship between Klaus and Caroline. Aired in February this year, the crossover garnered a lot of viewership.

- Sleepy Hollow and Bones:

One of the weirdest combinations on television, Sleepy Hollow and Bones pulled off a crossover episode that viewers could watch. The two shows deal with two completely different genres – Sleepy Hollow revolves around supernatural drama whereas Bones focuses on solving mystery by examining the remains of victims. However, makers intertwined the two storylines to bring together a decent episode.

- Arrow and The Flash:

The Flash and Arrow
Facebook/The Flash

While many shows have a crossover after they have established their storylines, The Flash viewers saw a crossover in the very first episode. A spin-off show from the shows, Arrow, Flash's first episode shows the lead character waking up from coma nine months later after being hit by a lightening, only to find out that he has become a superhuman. Coming to terms with the changed scenario, Flash immediately pays a visit to Starling City's Oliver Queen, or Green Arrow.

- Mad About You and F.R.I.E.N.D.S:

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While Mad About You was owned by NBC, F.R.I.E.N.D.S was aired by Warner Bros Television. The common link between the two shows was Lisa Kudrow. The actress played the role of Phoebe in F.R.I.E.N.D.S. whereas she was seen playing Ursula, a flaky waitress at Riff's Bar, a restaurant Paul and Jamie frequently visited on Mad About You. In one of the episodes of the first season of the WB show, as part of a night of NBC sitcom crossovers, Jamie and Fran walk into Central Perk and mistake Phoebe for Ursula. In another crossover instance, Jamie causes a city-wide power blackout in New York City, and the effects of the blackout are seen in the Friends episode.

- Bart Simpson and 24's Jack Bauer:

'The Simpsons Renewed For A 26th Season But Tragedy To Strike In Season 25
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Listed as one of the best Simpsons' episodes by The Guardian, the episode was promoted as the 400th episode of the series. Titled 24 Minutes, the episode is a spoof of the Fox television drama 24, and sees Principal Skinner's new Counter Truancy Unit (CTU), led by Lisa Simpson, attempting to prevent a stink bomb being released at Springfield Elementary School. Lisa asks Bart to help them. During his investigation, Bart's phone call is accidentally cross-connected with a call from Jack Bauer of 24, who is busy in a gun battle, and turns it into a prank call against him. The parody saw Kiefer Sutherland and Mary Lynn Rajskub as their characters from 24 participating in the iconic episode.