A RadioDrama on [[Creator/TheBBC BBC Radio 4]] which later became the basis for [[Franchise/TheHitchhikersGuideToTheGalaxy various spin-offs]]. It (technically) ran for twenty-six years and as many episodes.

It started with only two seasons in 1978-1980. It was later mixed-and-matched into the first two novels. It was later revived in 2003 and spawned three more seasons based on the last three books by Creator/DouglasAdams.

The cast was fairly consistent across the board, with only Peter Jones and Richard Vernon being replaced in the last three seasons due to their deaths in the interim. Even death didn't stop Creator/DouglasAdams from putting in an appearance in Seasons 3 and 5, despite the fact that he ''wasn't'' reprising a previous character.

!!The original two seasons contain examples of:
* ActingForTwo: Bill Wallis plays both Prosser (the man in charge of demolishing Arthur's house) and Jeltz (the alien in charge of demolishing Arthur's planet).
* DeadlyEuphemism: Played with.
** Slartibartfast threatens Arthur that, unless Arthur comes with him promptly, he will be late -- as in "the late [[TheNameIsBondJamesBond Dent Arthur Dent]]".
--->"[[DontExplainTheJoke It's a sort of threat, you see...]]"
** Hig Hurtenflurst explains his use of "revoked" to Arthur by spelling it out as "k-i-l-l-e-d". A subsequent episode reveals that this is part of a larger legal wrangle where (for various reasons) the representatives of a cloning agency were trying to get murder redefined in law. They'd managed to have the word legally changed, but not the spelling.
* GiantFlyer: The Brontitall. [[spoiler:Of the Deus Ex Machina Airlines variety. Not really ''that much'' of a spoiler.]]
* IdiosyncraticEpisodeNaming: Fit the First, Fit the Second, etc. This is a reference to Creator/LewisCarroll's ''Literature/TheHuntingOfTheSnark''.
** [[IdiosyncraticEpisodeNaming Idiosyncratic Season Naming]]: Primary Phase, Secondary Phase and Tertiary Phase, while idiosyncratic, make sense as a sequence.
*** Quandary Phase and Quintessential Phase, however, do not complete the pattern formed by the first three. The names were chosen because they are "less daunting, more memorable and are a bit easier to spell" than the correct Quaternary and Quinary.
* INeedAFreakingDrink: When it's established that (due to the effects of the Infinite Improbability Drive) Ford and Trillian are the only two members of the core cast who haven't coincidentally met before:
-->'''Zaphod''': Oh, god. ''[rapidly]'' Ford, this is Trillian, Hi. Trillian, this is my semi-cousin Ford who shares three of the same mothers as me. Hi. ''[...]'' Zaphod Beeblebrox, this is a very large drink. ''Hi.''
* LargeHam: Zaphod, played by Mark Wing-Davey, was written as such [[{{Typecasting}} specifically because he was played by Mark Wing-Davey.]]
* MessageInABottle: Arthur and Ford are stranded on prehistoric Earth, and attempt to attract the attention of a passing spaceship by waving a towel at it. A volcano then erupts, covering the towel with lava. When the Earth is blown up six million years later, the now-fossilized towel gets launched into space and found by Zaphod Beeblebrox in the spaceship Heart Of Gold, who travels back in time and rescues them. (Things like this tend to happen whenever you use the Heart Of Gold's "Infinite Improbability" drive.)
* RelaxOVision: During the approach to Magrathea where, supposedly in order to help combat rising stress levels in the galaxy, it was carefully explained to the audience that no one was going to get killed in the ensuing confusion -- although one unidentified person would be bruised on the arm. [[spoiler:It's Arthur.]]
* {{Retronym}}: It wasn't until the radio series were released on tape that the seasons began being referred to as Phases. (This only applies to the first two seasons, the rest were labeled Phases right out of the gate.)
* SecondEpisodeIntroduction: Zaphod, Trillian, and Marvin.
* ShapedLikeItself: Zaphod and Ford find themselves in a cave made out of marble, very slippery marble. Zaphod tries to compare it to the slipperiest thing Ford can think of. Unfortunately, the slipperiest thing Ford can think of is the marble, leading to the statement "This marble is as slippery as this marble."
* UnstableGeneticCode:
-->''The Haggunenons of Vicissitus Three have the most impatient chromosomes of any life form in the Galaxy. Whereas most races are content to evolve slowly and carefully over thousands of generations, discarding a prehensile toe here, nervously hazarding another nostril there, the Haggunenons would do for Charles Darwin what a squadron of Arcturan stunt apples would have done for Sir Isaac Newton. Their genetic structure, based on the quadruple sterated octohelix, is so chronically unstable that, far from passing their basic shape onto their children, they will quite frequently evolve several times over lunch.''
* VengefulVendingMachine: Arthur Dent, sick of getting a drink which is almost, but not quite, entirely ''unlike'' tea from the Nutri-Matic machine, gives it a lengthy lecture on the nature and history of real tea. The machine hijacks the starship's entire computing power to work on the problem of why on earth someone wants dried leaves in boiling water, leaving the ship defenceless against a missile attack. Arthur gets his tea in the end, though.
* WeirdTradeUnion: The Amalgamated Union of Philosophers, Sages, Luminaries and Other Professional Thinking Persons, which opposed using the computer Deep Thought to find the Answer to the Ultimate Question of Life, the Universe and Everything because, under law, the quest for Ultimate Truth was under their prerogative, and not the computer's. They even threatened to go out on strike, though they dodged the question of who, exactly, that would inconvenience.
* WhatDoesThisButtonDo: Lampshaded.
-->'''Arthur Dent''': What happens if I press this button?
-->'''Ford Prefect''': I wouldn't--
-->'''Arthur Dent''': Oh.
-->'''Ford Prefect''': What happened?
-->'''Arthur Dent''': A sign lit up, saying "Please do not press this button again."

!!The three revived seasons contain examples of:
* AndThereWasMuchRejoicing: The final fate of Wowbagger.
* BackForTheFinale: The ending sequence of the final episode of the Quintessential Phase (and the final episode of the radio series altogether) has the return of Fenchurch and Marvin, as well as Max Quordlepleen, the Great Prophet Zarquon, Wowbagger, Lintilla, Prosser, and a few other bit players.
* CastingGag: Geoffrey Perkins, Douglas Adams's boss at the BBC, plays Arthur Dent's boss at the BBC.
* ComicBookTime: The original two series, broadcast from 1978 - 1980, were intended as a contemporary piece. While the narrative quickly left Earth and there is not much to date it, it can still be a bit jarring when the [[Main/RecursiveAdaptation later three series]] have scenes on or in reference to Earth that make more modern cultural references, or include as common technology things that would not yet have been common or even have existed. Of course, when dealing with possibilities such as different versions of Earth existing across multiple planes of reality, one supposes that such things may be relative.
* ContentWarnings:
** One episode of the Tertiary Phase finishes with the warning: "The preceding program contains scenes of extreme violence which may be disturbing to some viewers. Time travellers of a nervous disposition may wish to consider listening to something else for the previous half-hour."
** Played straight when the episode featuring the destruction of the Guide offices was broadcast on [[Creator/TheBBC Radio 4]] [[TooSoon shortly after 9/11]].
* [[spoiler:EverybodyLives: All the core cast manages to survive at the very end of the series, including Trillian and her alternate self (who merge into a single being), Marvin (who was still under warranty when he expired), and Fenchurch (who had been waiting at Milliway's since she seemingly ceased to exist). And the Vogons fail to eradicate every Earth.]]
* OddlyNamedSequel: The original two series and the later adaptation of "Life, the Universe and Everything" were released as the Primary, Secondary and Tertiary Phases. The adaptations of the last two books became the Quandary and Quintessential Phases.
* RemakeCameo: Inverted--cast members of [[Series/TheHitchhikersGuideToTheGalaxy the remake]] play roles in the continuation of the original series:
** The Quintessential Phase series had Sandra Dickinson, Trillian in [[Series/TheHitchhikersGuideToTheGalaxy the TV version]], voice Tricia [=McMillan=] (Trillian's alternate universe counterpart).
** David Dixon, the TV Ford Prefect, also had a cameo in the second episode of the Quandary Phase, getting pissed off at Arthur for trying to donate to save the dolphins when he should know they've all vanished. (This case is lampshaded, as Arthur--still played by Simon Jones, who acted alongside Dixon in the TV series--asks if they've met before.)
** Chris Langham, Arthur Dent from the 1979 Institute of Contemporary Arts stage production of ''Hitchhiker's'', pops up as Prak in the finale of the Tertiary Phase.
** StephenFry, the [[Film/TheHitchhikersGuideToTheGalaxy film's]] Guide, as Murray Bost Henson.
* {{Retcon}}: The entire Secondary Phase is AllJustADream Zaphod had, to cover for the Tertiary Phase opening with Ford and Arthur still being stranded on prehistoric Earth.
* SoundEffectBleep: Fit the Sixteenth from the Tertiary Phase. The book ''Life, The Universe And Everything'' on which the Phase was based featured an award for "The Most Gratuitous Use of the Word Fuck in a Serious Screenplay"; since it was scheduled to be broadcast at 6.30 pm the word was still uttered by the actor but completely masked by the sound of a starship engine.