Where should someone start exploring 20th century avant garde composers? Let's assume the person is already into ambient, techno, IDM, radiophonics, and contemporary experimental musics.

So we're not looking for what you think are necessarily the "best" pieces, but rather composers and pieces that are good entry points for the open-minded and curious.

Preferably, recommend specific recordings/performances that are currently available in some form.

I'll start by suggesting John Tilbury's 1975 recoding of John Cage's Sonatas & Interludes.

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I think this represents a good, moderate introduction to Cage.

What would you suggest?

Music for 18 Musicians by Steve Reich but get the Nonesuch Edition that came out about 10 or 11 years ago. That performance is great.

Anything composed by György Ligeti is seriously worth the time.

Henry Cowell's "Pulse" is great as well for the more percussion oriented pieces.

I got started on Arvo Pärt with "Passio"—don't know if that falls strictly within the parameters you're asking, but it did eventually lead me to the 20th century avant garde.

The soundtrack for "I Am Love" features a bunch of John Adams pieces, and might act as a nice sampler set of his music:

I'm really an asker and not an answerer here, but I'll add that my entree into minimalism was the soundtrack to Koyaanisqatsi.

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Good starting points, like I wish someone recommended me years ago:

John Cage
Charles Ives
Edgard Varese
Moondog
Luc Ferrari
Karlheinz Stockhausen
Alvin Lucier
Robert Ashley
Then, only if you feel adventurous, Luigi Nono and Iannis Xenakis.

Minimalism:
Steve Reich
Terry Riley
Tony Conrad
Pauline Oliveros
Eliane Radigue
Rhys Chatham
Yes, you have Young, Glass, Nyman, Palestine - but I suggest to start from the above.

Electronics, to complement the Radiophonics:
Raymond Scott
Dutch mavericks - Dick Raaijmakers, Tom Dissevelt, and Henk Badings

Proto-Ambient:
Erik Satie
Toru Takemitsu
Basil Kirchin

Then some avant garde came out of Exotica (Denny, Esquivel, Baxter), cartoon music (Stalling), and soundtracks (Morricone, Mancini, Teiji Ito).

Great list, Fabio. Oliveros and Radigue I've already gotten into via my taste for drone and radiophonics.

A few to add to Fabio's excellent list:

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I second Fabio's recommendation of Basil Kirchin's "Quantum." I heard it first and still prefer it over his other works, but if you can track down a copy of "Worlds Within Worlds" (Parts 3 & 4), it's rightly considered a classic.

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For when you do get around to Xenakis:

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Okay, I ordered a copy of Music for 18 Musicians (the ECM version, actually). I really do need to delve into Xenakis. I've heard some of his stuff performed live and liked it--I just haven't researched it enough to know where to start.

I'm going to dig into some of these other suggestions this weekend, time permitting.

I would also like to mention Luciano Cilio in this branch. Anyone who's ever heard his masterpiece Dell'Universo Assente, reissued by Die Schachtel a few years ago, will probably nod in agreement; everybody else is in for a treat when they'll get to it.

He may not be considered a composer by some, and to my knowledge he didn't compose for others (his output is extremely limited anyway), but his atmospheric and emotionally charged minimalism seems to fit the bill here.

* nods in agreement *

One of my all time favorite performances is Ursula Oppens transfixing rendition of Frederic Rzewski's masterpiece The People United Will Never Be Defeated (amzn.to).

Atonal music typically is more intellectual than emotional. But Elliott Carter somehow manages to stir passion while stretching my ear.

Here's a link to Elliot Carter's String Quartet's (I couldn't figure out how to add this to my previous comment): amzn.to.

The list is already impressive, let me add some of the more accessible musicians from the post-Soviet and ex-Soviet Bloc territories: Arvo Pärt, Erkki-Sven Tüür, Giya Kancheli, Henryk Górecki, Galina Ustvolskaya, Alfred Schnittke.

Hmm, 20th century avant-garde... would Stomu Yamash'ta qualify? How about Leo Brouwer? [/disclaimer: not a music historian, just interested...]

Since no one suggested Györgi Ligeti I'll do the honors. Have you seen The Shining or other Kubrick films? Ligeti's music is used a lot in them. Take for instance his famous pieces Atmosphères and Lux Aeterna (used in 2011: A Space Odyssee):
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Ligeti:
en.wikipedia.org

I'm going to conclude this branch tomorrow, unless anyone has last minute comments. Thanks for all the great suggestions.

Weighing in late, sorry. The answers that have appeared here already are really solid, Fabio Lugaro's list in particular. Rather than re-emphasize Morton Feldman, Steve Reich, Pauline Oliveros, and a handful of others, I'l just list five whom, far as I can tell, weren't mentioned here previously:

- Sofia Gubaidulina
- Wynton Marsalis (especially Citi Movement)
- Ingram Marshall (especially Fog Tropes)
- Krysztof Penderecki
- John Tavener (especially Ikon of Light)

In the end, just start with a few and see where it takes you. Dive in too deep, you may just get disoriented and turned off.