History Main / RequiredSpinoffCrossover

** In CSI NY's second season, [[Series/{{CSI NY}} Mac Taylor]] went down to Miami and then returned to New York with [[Series/CSIMiami Horatio Caine]] as they tried to catch a suspect together.
** The original and CSI NY crossed over when Mac went to Las Vegas to surprise his girlfriend/future fiance and discovered she'd been kidnapped. When he and DB Russell realized she'd never left New York and was being held there, Mac returned to NY with DB in tow to find her before it was too late.
*** Incidentally, the original never crossed with the spinoffs until after Creator/WilliamPetersen's departure, because he was against the spinoffs from the start, believing it watered down the writer pool on the original.

See also PoorlyDisguisedPilot, where an episode of the parent series is used to introduce (or test the waters for) a spinoff, and SpinoffSendOff, a crossover in the pilot of the spinoff to establish the connection with the previous series.

See also SpinoffSendOff, a crossover in the pilot of the spinoff to establish the connection with the previous series.

* Although Mrs. C (Chica's mother) made occasional appearances on ''[[Creator/{{Sprout}} The Sunny Side Up Show]]'' prior to ''Series/TheChicaShow'', she and Mr. C still made guest appearances on the block.

* Arthur, D.W. and other regular ''Arthur'' characters would appear time to time on ''WesternAnimation/PostcardsFromBuster'', usually by phone, due to the premise of the show being that Buster is traveling across the world.

* After its second season began with a crossover episode between the two, ''WesternAnimation/MiloMurphysLaw'' would have Perry the Platypus from ''WesternAnimation/PhineasAndFerb make regular appearances. Meanwhile, Dr. Heinz Doofenshmirtz would become a full-on {{transplant}}, even moving into the main character's home.

* After its second season began with a crossover episode between the two, ''WesternAnimation/MiloMurphysLaw'' would have Dr. Heinz Doofenshmirtz and Perry the Platypus from ''WesternAnimation/PhineasAndFerb make regular appearances, with the former even moving into the main character's home.

* {{Averted}} with ''WesternAnimation/BeavisAndButthead'' and ''{{WesternAnimation/Daria}}''. While there were plans to do a crossover, it quickly became apparent that the drastically different tones of the two shows meant there was little audience overlap, and thus not much to gain from doing one. Nowadays many people don't even realize that the shows are related--the only reference to ''B&B'' in ''Daria'' is from the first episode, when Helen mentions that they moved to Lawndale from Highland, the fictional Texas town where ''B&B'' took place. Even {{Flashback}}s to Daria's childhood don't make the connection.

* WesternAnimation/{{SpongeBob|SquarePants}} makes a cameo in ''The Patrick Star Show''.

* ''WesternAnimation/TheCasagrandes'' has had Lincoln make cameos several times throughout the show. Most notably, "Cursed!" involved the Casagrandes visiting the Loud family.
* [=SpongeBob=] makes a cameo in ''The Patrick Star Show''.

* An interesting example of this happens in ''Series/TheNoddyShop''. Unlike other shows utilizing ImportationExpansion, Noddy, the star of [[WesternAnimation/NoddysToylandAdventures the show the series is a framing device for]], would frequently interact with the puppet and human characters created for the series.
* : Although Mrs. C (Chica's mother) made occasional appearances on [[Creator/{{Sprout}} The Sunny Side Up Show]] prior to ''Series/TheChicaShow'', she and Mr. C still made guest appearances on the block.

* : Although Mrs. C (Chica's mother) made occasional appearances on [[Creator/{{Sprout}}''The Sunny Side Up Show]] prior to ''Series/TheChicaShow'', she and Mr. C still made guest appearances on the block.

* {{Subverted}} with ''WesternAnimation/BeavisAndButthead'' and ''{{WesternAnimation/Daria}}''. While there were plans to do a crossover, it quickly became apparent that the drastically different tones of the two shows meant there was little audience overlap, and thus not much to gain from doing one. Nowadays many people don't even realize that the shows are related--the only reference to ''B&B'' in ''Daria'' is from the first episode, when Helen mentions that they moved to Lawndale from Highland, the fictional Texas town where ''B&B'' took place. Even {{Flashback}}s to Daria's childhood don't make the connection.

* {{Subverted}} with ''WesternAnimation/BeavisAndButthead'' and ''{{WesternAnimation/Daria}}''. While there were plans to do a crossover, it quickly became apparent that the drastically different tones of the two shows meant there was little audience overlap, and thus not much to gain from doing one. Nowadays many people don't even realize that the shows are related--the only reference to B&B in ''Daria'' is from the first episode, when Helen mentions that they moved to Lawndale from a place called "Highland." Even {{Flashback}}s to Daria's childhood don't make the connection.

** Creator/JonathanFrakes appeared in all three spin-off series (as William Riker in ''[[Series/StarTrekVoyager Voyager]]'' and ''[[Series/StarTrekEnterprise Enterprise]]'' and as Thomas Riker in ''[[Series/StarTrekDeepSpaceNine Deep Space Nine]]'').

* An interesting example of this happens in ''Series/TheNoddyShop''. Unlike other shows utilizing ImportationExpansion, Noddy, the star of [[WesternAnimation/NoddysToylandAdventures the show the series is a framing device for]], would frequently interact with the puppet and human characters created for the series.

* An interesting example of this happens in ''Series/TheNoddyShop''. Unlike other shows utilizing ImportationExpansion, Noddy, the star of [[WesternAnimation/NoddysToylandAdventures the show the series is a framing device for]]'', would frequently interact with the puppet and human characters created for the series.

*** One episode focused on Sarek, Spock's father. A later two parter featured Spock himself and a single episode had a visit from Scotty.

** ''Series/LawAndOrderCriminalIntent'' followed the same pattern, with Briscoe and Green called upon to leak info of a serial poisoner by Goren and Eames in one episode, Van Buren asked about a suspect that formerly served under her in another episode, and Skoda called in for a psychiatric opinion in yet another episode. And before ''L&O: CI'' launched, there were plans for a "mega crossover" between the three series centered around a terrorist attack...[[TooSoon but then 9/11 happened]].

* {{Subverted}} with ''WesternAnimation/BeavisAndButthead'' and ''{{WesternAnimation/Daria}}''. While there were plans to do a crossover, it quickly became apparent that the drastically different tones of the two shows meant there was little audience overlap, and thus not much to gain from doing one. Nowadays many people even realize that the shows are related--the only reference to B&B in ''Daria'' is from the first episode, when Helen mentions that they moved to Lawndale from a place called "Highland." Even {{Flashback}}s to Daria's childhood don't make the connection.

* ''Series/{{Angel}}'' had Oz and Spike showing up in one episode to follow up on plot from ''Series/{{Buffy|the Vampire Slayer}}'', Buffy showing up in "I Will Remember You," and Faith fleeing to LA after the events of "This Year's Girl"/"Who Are You?" with Buffy pursuing.
** Willow also showed up once to deliver news about Buffy, and again later when the AI team needed her to perform a spell.

** There was also a special crossover "event" between the ''Buffy'' episode "Fool For Love" and the ''Angel'' episode "Darla", in which several flashback scenes are seen from Spike's perspective in the ''Buffy'' episode and then again from Darla's perspective in the ''Angel'' episode which originally aired immediately afterward.

** The episode "Cyberwoman" features a partially converted Cyberman from the aftermath of the ''Doctor Who'' series 2 finale.

*** ''Voyager'' character Tuvok appeared in one of Deep Space Nine's AlternateUniverse episodes since the differences in timelines means that there was no reason for that version of the character to be trapped in the Delta Quadrant like the main universe version.