I'm mainly a Terran player, and I'm not sure what to do when my opponent is fast expanding.

I can try to kill the expo, but that usually fails due to the sheer weakness of marines when they're not behind a wall.

I can try to run straight for their main, but this usually fails due to ^

I can fast expand to my own natural, which sometimes works.

What should I do if:
a) Enemy Protoss FE
b) Enemy Terran FE
c) Enemy Zerg FE
?

http://day9tv.blip.tv/file/4486834/

Zerg FE: I hope you scouted early. Go early cc, 2rax all in, or 4rax all in. afaik.

You have essentially two options: timing pushes and expanding yourself, while harassing.

TvZ
- If you went 2rax before gas you can start rallying Marines towards his natural to try to do some damage. You can then build your second CC, still before getting gas.

TvP
- Depends on how soon he expanded, usually this is almost an auto win if you 2rax or 3rax push because he won't be able to do as much with forcefields and Terran bio > Gateway units.

TvT
- This is annoying. Pressuring is tough because of Bunkers so usually you're just stuck being behind. If you want to hit a Marauder/Marine timing push you can, but generally there's enough tanks out by that time for your opponent to defend. You can try to use Ghosts or Reapers to harass, if you like.

Pretty much as above. You either go all in, or expand yourself and try to match his economy. And the choice that you make really depends on what you've been doing up until you realise your opponent went for a fast expand, and to a larger degree, when you realise your opponent has gone for a fast expand.

Or just do what most Terrans do... MMM, stim, +1, +1 go. Gg

If you're a macro player, expand yourself.

If you're a micro player, reapers + nitro packs ASAP. Then expand + contain. Mutlipronged harassment is key. Keep them focusing on getting rid of you instead of increasing production while you increase production.