After 1. Nf3 I play d5. After 2. c4 I play c6. It fits with my choices against 1. e4 and 1. d4 (the Caro-Kann and Slav).
What do you play against the Reti (1.Nf3) and why?

Any 1. d4 defensive setup will do, as it's eventually going to transpose.
I usually go with 1... Nf6, aiming for a hypermodern setup for Black (KID, NID, or QID).

1.e6 With the following ideas: 2.e4 c5 3.d4 Nf6 4.e5 Nd5 5.c4 Nc7 6.Nc3 d5 , 2.d4 Nf6 3.c4 Bb4+ 4.Nc3 d5 , 2.d4 Nf6 3.e3 d5 , 2.c4 c5 , 2.g3 c5 , 2.Nc3 Nf6 3.d4 Bb4 and 2.e3 Nf6

I'm interested to know what you all play against the Reti and why.
I used to play 1...c5 hoping for a transposition to the Sicilian but this only happened once, most of the time, my Reti playing opponents play 2.c4. Thus I've decided to meet the Reti by playing 1...d5 and if 2.c4 then 2...d4 the so called "Reti Advance variation" that I noticed was scoring pretty well for Black. Which brings me to my other questions, does anyone play the Reti Advance variation (from either side), if so what do you think about it, and does anyone know of any books or videos that talk about the Reti Advance from the Black side? Also in the Reti Advance it looks to me as if Black gets more space out of the opening, so does this disprove what Kramnik said about the Stonewall Ducth being the only opening where Black gets more space?
1.Nf3 c5 2.c4 is the symmetrical English

I'm interested to know what you all play against the Reti and why.
I used to play 1...c5 hoping for a transposition to the Sicilian but this only happened once, most of the time, my Reti playing opponents play 2.c4. Thus I've decided to meet the Reti by playing 1...d5 and if 2.c4 then 2...d4 the so called "Reti Advance variation" that I noticed was scoring pretty well for Black. Which brings me to my other questions, does anyone play the Reti Advance variation (from either side), if so what do you think about it, and does anyone know of any books or videos that talk about the Reti Advance from the Black side? Also in the Reti Advance it looks to me as if Black gets more space out of the opening, so does this disprove what Kramnik said about the Stonewall Ducth being the only opening where Black gets more space?
1... c5 2. c4 is played 33% of the time and this can often be transposed into the accelerated dragon sicilian, or an improvement to it where black plays d5 in one move. This may not be your choice sicilian but it is a sicilian, and it's legit... More fun than playing some random line in the reti -
As you can see in the last board, if white doesn't play e4 at some point and transpose into the sicilian black just plays d5 in one move and is good to go, pushing d5 is what most of the strategy in the sicilian revolves around. Especially the dragon - if the center breaks up in the dragon and you're not facing a yugoslav attack (which is not possible in these lines) you are really golden.
Against 2. g3 or blacks other moves you can generally just play this double fiancetto setup, it's actually quite good -
As a sicilian player that's most of what you need to know against the Reti - it's not that bad once you know the first few moves you want to play.
To play these lines I went and learned the hyperaccelerated dragon as kind of a secondary weapon. Generally it's good for your chess to play a few different sicilians - the dragon setup is good to know, there are many sidelines in the sicilian that a dragon setup can handle very well.
This can also be used against the english, not entirely but in a large part - though there I feel the Kings English is too good to pass up. And it's also a reversed sicilian.
I have read all of your posts, and it is my pleasure, as someone who plays the Reti almost exclusively, to set the record straight. First of all the Reti is not uncommittal. See, most players play either 1.d4 or 1.e4 and play and know the traditional defenses to each. When I play 1.Nf3 I know I do not need to contend with any open king pawn games (GOOD!!), No Queen Gambit games (even better) and will not face The Benko, Nimzo, French, Sicilian, Pirc, Benoni, or Caro-Khan. Black is off line. Now, black can transpose into an English ( and if I allow it the K.I.D), but every Reti player knows that and is prepared to play the English at anytime, but he'd rather not. If white really wanted the English, he would have just played 1.c4 in the first place. I have played over 1000 Reti games and probably as many English games, and I can hardly remember if I ever opened with 1.c4! What the Reti player wants is either 1..d5, 1..e6, 1..c6, 1..Nc6, 1.Nf6, 1..g6, or 1..f5. What he doesn't want is 1..c5 or 1..b6. See, I want the Reti, not the English and I'd rather not trade light bishops. White can also transpose into a reversed Grunfeld, or the incredibly flexible K.I.A. If you really want to see how the Reti should be played at the highest level, check out games by GM Daniel Hausrath and GM Teimour Radjabov.
This is well put and generally I agree. b6 is a good response, neutralizing the bishop. That's why I like the double fiancetto, I don't want to play all of b6 but in specific lines it's good.

There seem to be many opinions but not simple answers to the simple question. 1.Nf3 is very versatile for white and can be transposed into a Reti, Catalan, K.I.A., Reversed Grunfeld or English by white. Black can attempt to transposed to any of his queen pawn games, such as: Queen's Gambit, Slav, Nimzo- Indian, Queen's Indian or K.I.D. The best move against 1.Nf3 depends on the player but 1.Nf3, 1..c5, 1..d6 and 1..b6 seem to work best. 1..d5 is probably the most popular but it's usually what the Reti player wants. Still, there's absolutely noting wrong with 1..d5. As someone who favors Reti lines, I'd say I least like to see 1..c5 or 1..b6. Why? Because 1..c5 will be either an English (if 2.c4 is played) or a Sicilian (if 2.e4 is played). If I wanted an English, I would have played 1.c4 and if I wanted a Sicilian I would have played 1.e4. In the opening, I like the idea of not playing to my opponent's strength. If black wants a King's Indian he can get it beginning with 1..Nf6. You can play anything accept 1.e5. It's your choice.
I agree the moves 1...c5,1...d6, and 1...b6 are playable.
However, The part I disagree with is you saying they work best.
The statistical facts do not support your statement.
After the move 1.Nf3:
The moves 1...d5 or 1...g6 statistically are doing the best.
You really need to know the lines and dig into the stats deeper before you make a statement like this. Looking at the stats on the first move and noting some 1% difference isn't the right way to analyze them.
But honestly white shouldn't base his response to the Reti on stats, really just on whatever his repertoire is.
There seem to be many opinions but not simple answers to the simple question. 1.Nf3 is very versatile for white and can be transposed into a Reti, Catalan, K.I.A., Reversed Grunfeld or English by white. Black can attempt to transposed to any of his queen pawn games, such as: Queen's Gambit, Slav, Nimzo- Indian, Queen's Indian or K.I.D. The best move against 1.Nf3 depends on the player but 1.Nf3, 1..c5, 1..d6 and 1..b6 seem to work best. 1..d5 is probably the most popular but it's usually what the Reti player wants. Still, there's absolutely noting wrong with 1..d5. As someone who favors Reti lines, I'd say I least like to see 1..c5 or 1..b6. Why? Because 1..c5 will be either an English (if 2.c4 is played) or a Sicilian (if 2.e4 is played). If I wanted an English, I would have played 1.c4 and if I wanted a Sicilian I would have played 1.e4. In the opening, I like the idea of not playing to my opponent's strength. If black wants a King's Indian he can get it beginning with 1..Nf6. You can play anything accept 1.e5. It's your choice.
I agree the moves 1...c5,1...d6, and 1...b6 are playable.
However, The part I disagree with is you saying they work best.
The statistical facts do not support your statement.
After the move 1.Nf3:
The moves 1...d5 or 1...g6 statistically are doing the best.