What do you play against it? I am not talking about an Anti-Marshall opening, I am talking about what to do after we enter the Marshall Attack.

According to the main line, Be3 is the best move, follows Bg4, Qd3, with the intention of Qf1. white can follow by developing the knight to d2, etc

I just don't play 1...e5 

When I find someone who does 1.e4 e5 I try to play Scotch

If you want to be booked up on your Ruy Lopez, good luck!

Here's something you might try: http://www.chesscafe.com/text/kibitz24.pdf

Some interesting stuff here http://www.chess.com/article/view/are-you-afraid-of-the-marshall-attack

You should not be going in for the Marshall Attack if you have to ask what to do on a public forum.  This is one of the most intensely analyzed lines of recent times, including by those at the highest levels.  The theory goes on for another 15 moves or so, more in some lines. 

If you are going to play this line successfully, you must stay up with the latest master games, available from TWIC in pgn format.  It's quite complex, and not for lower-rated players. 

Many lines have been analyzed well over 30 moves out whose final verdict is a draw. Honestly, if I were you, I'd play h3 instead of c3 just to get out of main line Marshall.

Well, I only liked Ruy Lopez because of the Marshall Attack. Then I should take your advices and play Scotch.

The Scotch can be tricky. But of course, there's a lot less theory than main line Ruys.

Ok, thanks for the advice..

It's pretty much up to you what you choose in the 1.e4 e5 2.Nf3 Nc6 arena. I choose scotch because the way I use it sits very close to Giuoco Piano and Two Knight's Defense lines that I am used to. I play Scotch Gambit and go into the d4 exd4 lines that way.

If you choose to play Ruy Lopez (3.Bb5) you are just going to be prepared to face those guys who are all booked up and ready for it.

Well, I'm not an expert about 1.e4 e5 openings but here's what I think:

Start with the Italian game (modern variation). It's typical piece placement and arising pawn structures are similar to the Ruy Lopez.

Resources: Start with Kindermann's excellent DVD. After watching this you're good to go. http://www.newinchess.com/The_Italian_Game-p-2468.html

If you need more analytical information check out Emms' book. http://www.newinchess.com/Beating_1_e4_e5-p-5026.html

After getting some experience with this opening start experimenting with the Ruy. But before doing so it could be helpful to read a book with verbal explanations about it. I'm recommending McDonald's latest effort http://www.newinchess.com/The_Ruy_Lopez__Move_by_Move-p-7015.html

And a small reminder at the end. You should aim to get positions that you've a clear idea about how you should continue. An equal position that you know your typical plan(s) is preferable to an advantageous position that you don't know anything about. If I were you I'll avoid Marshall (for both sides) like plague until I become a master.

 This isn't really answering the question since the OP asks right in the beginning what do you do after you're already in the Marshall Attack. But EdwardT2 does have a point: after 15.Be3 or Re4, the book lines go on forever. So I guess if you want to stick with this thing, you've got a lot of homework. One of the few times I faced this line, I tried chopping off the powerfully posted knight, 15.Bxd5 (very painful to give up the bishop) and then neutralize the white square attack with 16.Qf3, but after 16...Bg4 17.Qxd5 Rae8, my undeveloped back rank was a problem, and I got killed quickly. Maybe white can live through this mess with 17.Qg2......

I've always seen it go 15. Qf3, Bg4  16. Qg2  But that's the end of what I know...

To play Marshall attac:

http://www.chess.com/tournament/the-marshall-attack

If you find the best line please let Anand, Carlsen, Kasparov and everyoen else know,...

I think melvinbluestone's 15.Bxd5 is a safe way of meeting the Marshall. I would probably choose the same line.

15. Re4 is an interesting alternative.