As white, I often encounter the Philidor defense by black:
[FEN ""] 1. e4 e5 2. Nf3 d6
I usually play
d4 on the next move, but is this the best move? What are some lines (advantages or disadvantages) against the Philidor?
A good resource to study would be shredderchess.com's opening database.
To complement Daniel's answer: Black often aims for the solid Hanham setup with
If black plays
Black can try
The best way to reach the Hanham is probably with the radically different move order
Against the Philidor you might experiment with a line I developed myself (don't know how good it is), but if they surrender the center (i.e. exd4), then I develop a setup where I fianchetto the dark-squared bishop on b2, play f3 at some early point (with perhaps a later f4). I usually stick the other bishop on d3 then if they chop it with a knight I take back with the c-pawn, getting a better center than Black at the cost of bishop vs. knight. I usually can start up some sort of middlegame kingside pawn majority attack as a result of all this and create mate threats by Qe1-g3 along with the bishop at b2. It seems to work fairly well but I'm sure some IM/GM could refute the whole thing. Not like we regularly play against those guys though. :)