IOSYS is a prolific {{doujin}} circle that specializes in music remixes, particularly vocal remixes of the background music from the ''VideoGame/{{Touhou}}'' series, with 33 albums (as well as several karaoke/alternate-remix and best-of albums and two re-releases) just dealing with ''Touhou'' in the past six years. Many of their vocal remixes have been given [[FanVid music videos]] by various artists. They also make plenty of original songs as well, including songs for rhythm games.

IOSYS's (Touhou) albums to date:

* Touhou Kazakuraen ~ Phantasmagoria mystical expectation (May 2006)
* Touhou Otomebayashi (August 2006): Features IOSYS's biggest (and perhaps most memetic) song "Marisa Stole the Precious Thing"
* Touhou Tsukitourou (December 2006): Features IOSYS's ''second'' biggest song "It Stops at the Affected Area and Immediately Dissolves ~ Lunatic Udongein" (a.k.a. "Overdrive")
* Touhou Eijanhou (May 2007): Remixes of the stage and boss themes from ''Imperishable Night'' (except the Extra Stage, for some reason)
* Touhou Suisuisuusuu (August 2007)
* Touhou Kasokusouchi (December 2007)
* Touhou Makashinsai (May 2008)
* Touhou Souyuu Shinpi (August 2008)
* Touhou Hyousetsu Kashuu (November 2008): Remixes of fan-favorite Cirno's {{leitmotif}} (and, appropriately, their 9th album)
* Touhou Bubbling Underground (December 2008)
* Touhou Chotto Matten (March 2009)
* Bloom (March 2009): Solo instrumental remixes by IOSYS's guitarist, ARM
* Touhou Nengara Nenjuu (August 2009)
* Touhou [=JeuXinTerdiTs=] (December 2009): Like ''Bloom'', but with IOSYS member minami taking the spotlight
* Touhou Ageha (February 2010)
* Touhou Crystallized Ocean (March 2010)
* Moero! Touhou Brass Band (May 2010): Remixes of boss themes done in the style of high school baseball songs (re-released in August 2012 with a live band as "Moero! Touhou Brass Band Raw")
* Touhou Faithful Star (July 2010): The follow-up to ''[=JeuXinTerdiTs=]''
* Touhou Koiichigo Musume+ (August 2010)
* Touhou Merry-Go-Round (September 2010): Remixes of themes from ''Embodiment of Scarlet Devil'' done in the style of, well, merry-go-round music
* Touhou Fushigi Kakumei (October 2010)
* Touhou Unfufuufu (December 2010)
* Touhou Meringue Shoujo Yakou (March 2011)
* Touhou Variable Spellcaster (May 2011): The follow-up to ''Faithful Star'' (i.e., yet another solo minami work)
* Touhou Ageha Destiny (July 2011)
* Touhou Precious Ryuusei Shoujo (August 2011)
* Touhou Reflection of a Drive (September 2011): The follow-up to ''Variable Spellcaster'' (yes, ''more'' minami)
* Touhou Telepathy Shoujo Kashuu (October 2011): Remixes of Satori Komeiji's theme (like ''Hyousetsu Kashuu'', but with no drama tracks)
* Touhou Electric Denpa Shoujo (December 2011)
* Quality Underground (April 2012): Mostly solo-arranged by IOSYS member uno
* Touhou Twilight Heaven (April 2012)
* Touhou Ageha Nightmare (May 2012)
* Quaky Divines (September 2012): Dance-style remixes of various themes from ''Mountain of Faith'' (mostly solo-arranged by uno again)

IOSYS is also known for the song "FOE", a remix based around the infamous [[BossInMookClothing bosses in mook clothing]] from ''VideoGame/EtrianOdyssey''. They provided one of the endings for the anime ''[[Manga/PenguinMusume Penguin Musume Heart]]'', and many of the songs for ''Anime/YumekuiMerry''.

See also {{Albatrosicks}} (a side project created by members ARM, quim, and miko to do their own stuff), {{monoROSETTA}} (member void's side project), and SugarHoneyBabies (IOSYS' "original idol unit").
!!Tropes that appear in IOSYS's work:
* AlternateCharacterInterpretation[[invoked]]: IOSYS, being deeply ingrained in Touhou fandom, tends to take many of the same liberties with characterization as the print side. In particular, they are virtually the TropeCodifier for the "{{Tsundere}} Alice" characterization, thanks to remixes such as "Marisa Stole the Precious Thing", "Alice --> Sweet", and "Doll's Play".
* AudioAdaptation: ''Touhou Hyousetsu Kashuu'', ''Touhou Bubbling Underground'', and ''Touhou Chotto Matten'' each contain three or four drama tracks that either tell a continuing story (in the case of ''Touhou Hyousetsu Kashuu'') or shorter, independent ones.
* ClumsyCopyrightCensorship: [[AvertedTrope Averted]], for the most part. IOSYS re-released its ''Touhou Tsukitourou'' album from 2006 as ''Touhou Tsukitourou [[LampshadeHanging Safe!]]'' in 2009. The album had three new tracks, drawn from other IOSYS sources (including a previously limited-issue track), while removing three others. The three removed? [[AffectionateParody Parodies]] of songs from ''Anime/SaintSeiya'', Japanese singer Yousui Inoue, and LightNovel/HaruhiSuzumiya (the below-mentioned "Kire Kire Mayoi"). The album doesn't suffer from the removal.
* ConceptAlbum: ''Touhou Bubbling Underground'' revolves around Marisa and Nitori club-hopping in the underground hell.
* DerangedAnimation: [[ "It doesn't stop at the affected area, but goes deep inside and Aah Aaahn ~ The Final Udongein"]], a hardcore remix of Reisen's {{leitmotif}}. The two below-mentioned "Ecstasy" videos [[ also have this]] [[ going on.]]
* FilkSong: What IOSYS's work can be considered as, in a broad sense. In a more narrow sense, a number of their songs are in the parody-filk style -- see [[{{Trivia/IOSYS}} the Trivia page]] for a list.
* FunnyBackgroundEvent: [[ Well, funny and subliminally nightmarish]].
* GratuitousEnglish: Not really any more than most other Japanese pop-music groups, but credit goes to "[[ Special Ability to Sing a Love Spell]]", whose GratuitousEnglish is to the point that you ''can't even distinguish it as such [[AllThereInTheManual without the CD liner notes]]''.
** Also of note is "Reason to Fly", for having only the first two words being comprehensible.
** "Marisa Stole the Precious Thing" deserves mention for having gratuitous English, Chinese, ''and'' German all in the first verse!
* IntercourseWithYou: "Border of Extacy" and "Ecstasy Masochistic" take this to over-the-top levels.
** CovertPervert: [[spoiler:"Border of Extacy" is Youmu's imagination running wild with what Yuyuko does with Yukari]].
* [[WordSaladTitle Kanji Salad Album Title]]: The kanji for ''Chotto Matten'', ''Unfufuufu'', ''Souyuu Shimpi'' were randomly chosen for their pronunciation, and make absolutely no literal sense whatsoever. To date, only one of these titles, ''Souyuu Shimpi'' has had a literal translation done to show how non-sensical they are (''Eastern Thoughtfully Deep Forest Chicks'').
* LastNoteNightmare: "Blue Cirno", "Marisa Stole the Precious Thing".
* LongTitle: Several, which only get longer when the songs are themselves remixed, but the prize goes to the Website/NicoNicoDouga [[AffectionateParody parody]] "[Singing Attempt] The Brides that Queue Up the Affected Area are Impending the Precious Sun, Perfect Ignorant Fools! [Prismriver]", a medley of several of IOSYS's most popular songs, as performed by the Prismriver sisters.
* [[{{Memes/Touhou}} Memetic Mutation]]: IOSYS's more popular works, in addition to contributing to the aforementioned AlternateCharacterInterpretation, tend to generate memes within the Touhou community.
** Not just within the Touhou community, FOE!!
* MohsScaleOfRockAndMetalHardness: Most of their songs rate 4 on this scale.
* MoodWhiplash: Considering that is a group that often experiment with different styles of music, it's expected to happen.
** One of the examples is in ''Touhou Makashinsai'', where the melancholic and [[TearJerker tear-jerking]] song "Pray" is followed with the [[IntercourseWithYou pervy]] "Border of Extacy".
** Other example is in ''Touhou Otome Bayashi'': The first song is the [[DarkerAndEdgier dark and moody]] "Border of Death" following with the happy-upbeat "Marisa Stole the Precious Thing".
* MotorMouth: [[ Murasame United]] has some ''insanely'' fast rapping, in one segmenthh starting at 1:07 and again at 1:55.
* RearrangeTheSong: Pretty much their forte.
** ''Gossun Remix Zwei'' is an album made entirely of rearranges of ''Marisa Stole the Precious Thing'', from genres going from HeavyMetal ([[ S.S.H. - Detroit Marisa City]]) to HardcoreTechno ([[ DJ TECHNORCH - Black Miko Miko Meets Gabba]]).
* ShoutOut: Reimu lists off lots of fictional currency in "[[ Donations <3 Please]]," including [[VideoGame/RomancingSaGa Keros]], [[Anime/KirbyRightBackAtYa Dedens]], [[Franchise/FinalFantasy Gil]], [[VideoGame/MegaManBattleNetwork Zenny]], and [[VideoGame/MapleStory Mels]]. (She also mentions [[Franchise/TheLegendOfZelda Rupees]], but [[RealityIsUnrealistic they're the actual currency of India]].)
* ThemeNaming: Each of IOSYS's album names (until ''Bloom'') contains a kanji that refers to a character from the series ''Manga/RozenMaiden''. See the [[ Touhou Wiki]] page for more information.
** The group name itself might as well be named after a file named ''io.sys'' in the Windows OS. Plays nicely to their official website, which is named ''''
* YuriGenre: A few of their music videos, most notably "Marisa Stole the Precious Thing". The {{yuri}} level is also fairly prominent in [[ "Remirya@Full Moon"]].