A show which suffers an excessive regular turnover in the main cast. Regulars might last for a season or two at the most and even the (seemingly) most vital characters have a very high chance of being dropped at the end of the year. This is not merely LoadsAndLoadsOfCharacters - the core cast at any one time might be pretty small, just prone to being replaced.

Generally this happens most organically where a series is mostly concerned with a place of work such as a [[{{ER}} hospital]] or [[Series/TheBill police station]] at which some turnover might be expected, but it is not limited to such. For obvious reasons this is much more likely to happen in a LongRunner; Long-running shows without turnover frequent enough to qualify for this designation might still qualify for LongRunnerCastTurnover.

The actual way the old characters are written out can range from merely PutOnABus to BrotherChuck. Expect a lot of SuspiciouslySimilarSubstitute style replacements. When a new regular conveniently shows up just as another is shown the door, that's a ConvenientReplacementCharacter.

In music, this is RevolvingDoorBand
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!!Examples:

'''Based around a place of work:'''

* ''[[SavedByTheBell Saved By The Bell: The New Class]]'' dropped three of its seven regulars at the end of its first year (including the main character). It did the same thing the following year, including a survivor of the first season. And so on, and so...
** If you're interested:
*** Season 1 began with Mr. Belding, Megan, Weasel, Vicki, Scott, Lindsay and Tommy D.
*** For season 2, Weasel, Vicki and Scott were dropped and replaced with Screech, Brian, Bobby and Rachel. This was due to the six students in season one merely being [[{{expy}} expys]] of the original cast, and ''Saved By The Bell: The College Years'' getting cancelled led to Dustin Diamond returning.
*** For season 3, Megan, Brian and Bobby left, and R.J., Ryan and Maria arrived.
*** For season 4, Tommy D, Lindsay, and R.J. were dropped and Eric, Nicky and Katie arrived.
*** For season 5, Rachel was replaced by Liz.
*** For season 6, Ryan was replaced by Tony. Seasons 6 and 7 were filmed concurrently so there were no further cast changes on the show. Only Mr. Belding was on the show from beginning to end.
* ''DreamTeam'', the British soap opera set in a soccer club, featured dozens of characters during its ten year run, the great majority lasting a year or so. And an ''absurdly'' high death toll for a soccer club.
** Partially justifiable, in that much like in real life, players would sign for the club, before being sold on in a transfer window. Or, more than likely in the show, die.
* ''LawAndOrder'' and its assorted spinoffs.
* Seminal British police drama ''Series/TheBill''.
* ''{{ER}}''. Even Doctor Carter eventually left the show (as a regular). And during two or three seasons (with exception of some occasional episodes), NONE of the original series regulars were there. Curiously, ALL of them returned (briefly, mostly for only one or two episodes) for the last season.
* Any David E. Kelley show. Later seasons of both ''[[Series/AllyMcBeal Ally [=McBeal=]]]'' and ''ThePractice'' had older cast members being replaced by younger, sexier versions because either the actors would quit or Kelley would just get bored with their characters. ''Series/BostonLegal'' and ''Series/BostonPublic'' were the worst offenders as literally every season of both shows were littered with disposable characters that never gelled with the rest of the cast. Sometimes cast members would even get replaced mid-season.
* While the core group of interns in ''GreysAnatomy'' has remained constant until recently, there has been a constant parade of attending surgeons and new interns. In the later seasons so many new characters appear and then rapidly disappear that the show appears to be just throwing characters at the audience to see who sticks.
** As of season 9, the most important characters are 3 out of the 5 original main characters ([[RealLifeWritesThePlot due to real life circumstances]]) and {{Ascended Extra}}s who have been on the show for at least 3 years. The new interns rarely come back.
* ''{{Spooks}}'' has run for ten years and kills off or writes out main characters frequently and with extreme prejudice, something that starts in episode 2. Of the original cast only one remains, but he's more of the boss over the other main characters.
* The process is slower than in other examples, but ''Series/{{Degrassi}}'' has gradually retired its early characters in favor of new ones. Snake is the only one who's been a main character the whole time, and he's usually a minor character.
* ''{{Series/Damages}}'' tends to revolve around this trope with the appeal of the show generally being that most of the guest stars (and series lead Glenn Close) are film actors (William Hurt, Martin Short, John Goodman etc.) or other actors who normally don't do TV. In a given season these actors usually play the BigBad or other characters that are somehow tied up in the plaintiff's case. At the end of the season a case is usually resolved and most of the supporting cast are either killed off or [[PutOnABus put on buses indefinitely]].
** Season 1: [[spoiler: Ray Fiske and David Connor]] are both killed off (though the latter is a ForgoneConclusion).
** Season 2: Arthur Frobisher is demoted to guest star. Claire Maddox, Daniel Purcell and Wes Kruik are added to the cast, all get put on buses by seasons end.
** Season 3: [[spoiler: Tom Shayes]] is killed off. Joe Tobin, Marilyn Tobin and Leonard Winstone are added to the cast. All three, plus Arthur Frobisher are either killed off or PutOnABus at the end of the season.
** Season 4: Howard Erickson and Jerry Boorman are both added and PutOnABus at season's end.
** Season 5: Channing [=McClaren=] added.
* Brazilian soap opera ''[[http://en.wikipedia.org/wiki/Malha%C3%A7%C3%A3o Malhação]]'' has had a lot of this since [[LongRunner its inception in 1995]] - even changing the setting (from a gym to a high school, then another high school) to justify it. The actor with the longest run (6 years) even joked he and the runner-up (5 years) stood so long because they never scored with anyone in the soap opera.
* ''{{Casualty}}'' has old cast members leave and new ones join at least once a season on average. As of August 2012 (Season 27) only one character, Nurse Charlie Fairhead, remains from the first season.
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'''Not based around a place of work:'''

* ''Series/SabrinaTheTeenageWitch'' is a fairly mild case; only two main characters lasted to the end but most other characters stayed on the show for two to three years.
* ''{{lonelygirl15}}''. Danielbeast is the only character who was on the show from start to finish, and even he had some short phases of being PutOnABus.
* ''Franchise/PowerRangers''. Fairly justified in newer seasons that are actually completely different teams, but for the first several seasons, the main cast remained more-or-less consistent... except when it didn't, starting with the replacement of the red, yellow, and black rangers in season 2.
** Let's review anyway:
*** [[Series/MightyMorphinPowerRangers Season 1]] begins with Jason, Zack, Billy, Kimberly, and Trini. Tommy joins midway through.
*** [[Series/MightyMorphinPowerRangers Season 2]]: We have Rocky replacing Jason, Adam replacing Zack, and Aisha replacing Trini, all midway through the season.
*** [[Series/MightyMorphinPowerRangers Season 3]]: Katherine replaces Kimberly midway into the season, and Tanya replaces Aisha [[LastEpisodeNewCharacter at the end of the season]].
*** [[Series/PowerRangersZeo Season 4]]: There are only five Rangers, so Billy steps down, leaving Tommy, Adam, Rocky, Tanya, and Katherine. Jason temporarily returns midseason, before departing again at the end of the season.
*** [[Series/PowerRangersTurbo Season 5]]: Between seasons 4 and 5 (The ''Turbo'' Movie), Justin replaces Rocky. Midway into season 5, Tommy, Adam, Tanya, and Katherine are ALL replaced with T.J., Carlos, Ashley, and Cassie, respectively. At the end of the season, everyone heads into space on a rescue mission except Justin, who stays behind to be with his father.
*** [[Series/PowerRangersInSpace Season 6]]: Andros appears and gives the team new powers, and then they begin replacing the ENTIRE cast each year, starting with [[Series/PowerRangersLostGalaxy Season 7]].
* ''Series/BlakesSeven''. Originally Blake, five other people and a computer. By the fourth series it was no Blake, five people, only two of whom were in the first series, and two different computers.
* Soap opera ''Series/{{Neighbours}}'' (and probably others of its ilk) is known for having a small cast of longer-running (usually older) characters and a substantial contingent of high-schoolers with a very high turnover rate.
* ''Series/{{Angel}}''. At the end of the show, two of the original three characters [[spoiler:have died]], and at least one new character was accumulated every season.
* ''Series/DoctorWho''. The new series replaced at least one primary cast member every single year [[spoiler:until series 6]].
** In the 1960s, there were fairly frequent cast changes in calendar terms, though so many stories were done per year early on that short-term companions stayed through a significant number of episodes. Sara Kingdom and Katarina were both regulars for part of only one story, ''The Daleks' Master Plan''.
** In a few cases (like season 1 of the new series), [[TheNthDoctor the Doctor]] ''himself'' didn't last more than the one season in his current incarnation.
* ''Series/{{Lost}}'' is notorious for its AnyoneCanDie mentality that results in ''at least'' two main character deaths every season (except the first), plus several new regulars each year:
** Season 1: [[spoiler: Boone]] died and [[spoiler: Walt]] was kidnapped/PutOnABus
** Season 2: Ana Lucia, Eko and Libby join the cast. [[spoiler: Shannon, Ana-Lucia, and Libby]] all died, the latter two as part of a double murder. [[spoiler: Michael]] was PutOnABus.
** Season 3: Ben, Juliet, Desmond, Nikki and Paulo join the cast. [[spoiler: Eko and Charlie]] both died. [[spoiler:So did Nikki and Paulo, who were series regulars for the first 14 episodes, but no one cares about [[TheScrappy them]]]].
** Season 4: Daniel, Charlotte, and Miles join the cast. [[spoiler: Michael]] returned from his bus trip [[BackForTheDead and died]]. [[spoiler:Claire]] wandered off into the jungle and vanished.
** Season 5: No one joins the cast, though Ilana and Caesar were almost regulars; [[spoiler:Caesar dies however]]. [[spoiler: Charlotte and Daniel]] both died. [[spoiler:Locke]] dies, but [[spoiler:Terry O'Quinn]] stays on as [[spoiler:Jacob's nemesis]].
** Season 6: Recurring characters from previous seasons [[spoiler:Frank, Richard and Ilana]] join the main cast, [[spoiler:Juliet]] dies, and [[spoiler:Claire]] returns. [[spoiler:Locke, the Oceanic Six and various other characters]] also appear [[spoiler:as alternate timeline selves]].
* The TV series ''Series/MissionImpossible'' (1966-73) was one of the earliest examples of this; only Barney (Greg Morris) and Willy (Peter Lupus) were regular characters for the entire show's run. There is only one case of two consecutive seasons without at least one major character being replaced (2nd to 3rd).
* ''Series/EarthFinalConflict'': The villain Sandoval (Von Flores) was the only character who stayed throughout the whole run.
* ''Series/{{Heroes}}'' was meant to switch out most of its main cast every season, but this was changed. Instead, everyone from season 1 has immunity from death or bus trips ([[spoiler:Nikki]] died but [[spoiler:Ali Larter]] played a new character, [[spoiler:Nathan]] stuck around for half a season after [[spoiler:his]] death). Even recurring characters that weren't promoted, like the Haitian, get this immunity. Meanwhile, every volume introduces at least a dozen characters that appear in most, sometimes all episodes and are all replaced with a new set of characters. The only one that stuck was, ironically, introduced as one of a few one-off villains in the early half of his first volume.
* Due to the circumstances surrounding its creation, this occurred with ''Anime/{{Robotech}}'', which changed casts whenever it began adapting a new anime and thus began a new cycle. Twenty-one years later, when ''WesternAnimation/RobotechTheShadowChronicles'' was produced, it replaced the cast ''again'', combining a handful of characters taken from all three cycles with several new ones.
* Any superhero team tends to fall under this.
* ''Series/{{Skins}}'' switches out its entire cast every two seasons.
* ''Series/{{Farscape}}'': Following [[spoiler: Zhaan's]] departure in Season 3, at least one character was replaced with a new character each season.
* ''[[Series/TwentyFour 24]]''. Only Keifer Sutherland has appeared in every episode, with the runner-up (Mary Lynn Rajskub) lagging behind by about fifty episodes. Some get jettisoned between seasons (Day 4 and Day 7 ditched most of the main cast except for Jack Bauer), but for the most part, characters departed the show [[AnyoneCanDie by getting brutally killed off]]. Of the Main and Recurring characters from Day 1, only a tiny handful are still alive by the end of the series run, and only Jack Bauer is still actually ''in'' the show.
* Series/DragonsDen changes the Dragons every season or so, with at least one investor leaving per series. Indeed, out of the whole cast, only Peter Jones and Duncan Bannatyne have stayed for all eleven seasons, with everyone following on from Rachel Elnaugh leaving after only a couple of seasons.
* ''Series/{{Misfits}}'' lost all its original regular cast members over the third and fourth seasons. The fifth and final season had none of the original main characters.
* ''Series/TheWalkingDead'' has headed towards this thanks to AnyoneCanDie. Currently of the original 7 cast members only [[spoiler: 3]] are still alive as of the middle of season 4, two recurring characters from the first season who were later added to the main cast are still around and only two of the characters added in season two are still alive. A good number of characters from season 3 are still around though given then the low life expectancy of the source material one probably shouldn't expect them to stay safe for much longer.
* ''Series/{{Homeland}}'' may be headed towards this. As of the end of season 3 other than [[spoiler: Carrie and Saul]] the entire starring cast of the first season has either been killed off or PutOnABus.
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