What do I do when an opponent builds a refinery in my base?

Against Terran specifically, you should attack the SCV as it is building since that is the weak link. It will go down quickly and then you will not have to worry about it any more.

Zerg players using this tactic will often cancel the extractor as it gets close to finishing and start it over again, to maximize how annoying it is since the health of the extractor resets to maximum every time. In this case, at least he cannot grab both geysers.

Protoss players can build assimilators on both geysers fairly easily, but it's at a cost of 75 minerals each so you will be able to get out an extra zealot and a half that he won't have access to, giving you a slight army advantage first until you get the assimilators away.

Overall, the main thing you have to do against this kind of tactic is make sure that it doesn't affect your macro. Keep building probes nonstop to mine minerals as fast as possible, and possibly go for a 2-gateway or more build to pump out zealots as fast as possible until you've dealt with the issue.

Making my earlier comment into an answer.

If you are losing both gas to your opponent you are getting an assimilator way too slow. You should build 14/15 probes, then a pylon, then scout your opponent's base with that probe. Build another 3 probes (your pylon will be done by the time the 15/15 probe is done) then boost out the 11 and 12.

Build your gateway and as soon as you are able to after your gateway is started, build your first assimilator. If your opponent scouts you with one SCV/Probe/Drone he'll only manage to steal one gas from you worst case. Don't bother taking probes off the line unless he makes the mistake of moving his scout into your lines, then quickly select all probes and attack move. This will surround the SCV and kill it and your opponent will lose the scouting advantage and you won't waste time chasing him by doing this.

Once your gateway is done you can throw down a Cybernetics Core and when thats done you'll have enough gas to start your first stalker and Warpgate technology. This stalker can easily take out the gas that was stolen from you. Although you don't need double gas this early game, you want to focus heavily on minerals first and 3 probes on 1 gas.

Also, build your buildings out of sight of the stolen gas, so your opponent can't see what is going on in your base from his building's line of sight.

I actually dislike almost every answer thus far, as they all get at pieces of what's going on, but even together too much is missing.

Let's start with the cost of a gas steal. If we assume that this is a scouting worker that would not have been mining anyway then the costs are as follows:

• Protoss -> 75 Minerals
• Terran -> 75 Minerals + 30 Build time (which if your worker would have survived is 30 minerals)
• Zerg -> 75 Minerals + 20 Build time for new drone (which if your worker would have survived is 20 minerals)

So we can see that for Terran this is actually a more costly strategy. What's more if he wants to monopolize your gas you're looking at even higher costs for Terran and Zerg.

Now if a gas steal is so expensive, why are people doing it? There are a couple reasons:

• Prevent early cheese (Banshee, Reaper, Void Ray)
• Force you down a certain build
• Scouting purposes

For Zerg that third one is most common, and if you're not cheesing I suppose you don't have to worry about the first, so let's focus on that second one, as its the really dangerous one. If you're playing Terran (for example) and a Zerg steals both your gas, its likely you're going to see some quick Roach or Baneling play. So take this into account.

The most reasonable response to a Gas steal, is to take your other Gas. In a worst case scenario, you've wasted as much on Gas extraction as he has. Now don't go overboard and start building gas at 13 on every game, only when he takes one of your gas.

So what if he takes both? In your example the Terran player spent 210 Minerals on gaining a Gas monopoly. As a result he can't easily mount a significant offensive. In this situation, Chrono boosting out a couple Zealots to clear your gas, while using your excess minerals to get a Cybernetics Core and possibly move into 3 Gate is a perfectly reasonable response.

He's seriously handicapped himself; your strategy is largely unaffected, and you'll be in good position once your gas starts up. In this scenario he's really at a disadvantage.

Now you might consider, what happens if he sends a Reaper? Early Reaper play requires a significant economic sacrifice, as does a Gas steal, so the presence of one will necessarily delay the other. If he's stealing both your gas, he doesn't have the economy for a super fast Reaper. Now he can still build one, but it probably won't arrive till you have your Core and a couple Zealots up (and probably the Refineries down).

What people do need to watch for, in a gas steal, is something like Protoss stealing both gas vs Terran. This will naturally force Terran into a more Marine centric build which the Protoss can exploit with a Zealot heavy Immortal timing push. In that scenario I would suggest throwing down extra Barracks (without add ons obviously) and pumping out Marines to clear out the gas. These extra Barracks can then get Techlabs, and you should be able to field a decent sized Marauder/Marine force by the time he arrives.