What is the best defense against Marshall Attack? I have usually found the Marshall Attack in the Ruy Lopez to be very potent and very hard to counter.

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1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7 6. Re1 b5 7. Bb3 O-O 8. c3 d5


Are there any proven defensive lines or variations to use against this?
Just point me in the right direction, please.
A huge amount of theory has just been discovered in the Marshall Attack (I'm assuming you mean the line in the Ruy Lopez). However, this variation has recently fallen out of practice among top level GMs because white has been hard-pressed to find decent replies to the most critical lines. Basically, I would recommend playing one of the anti-Marshall games (8. h3 has been popular) unless you want to spend days studying opening theory to reach a suspect position with white.

Wikipedia has an article on the Marshall attack which is worth reading.