Harassment

Harrassing, in the context of SC2, is tickling your opponent with a few of your forces in a way that causes a disproportionate response. It's a good way to either gain or solidify an advantage, as your opponent will often overreact to the harass and open up a window for you to exploit, or he'll under-react and you can do a lot of damage with the harassing unit. Here are some tips for improving how you use harassment in your game.

1) Harassment is MORE powerful later in the game. Your opponent has to manage more and more things as he gets more bases and his army grows. At 13 food, attacking the SCV building your opponent's barracks isn't a big deal: he's got a bunch of free apm, since all he has to do is hit 's' once every once in a while. At 130 food, he has to macro and control a large army in addition to dealing with your harassment, so you've got a better chance of doing some damage.

2) If you can, do something else while you harass to take advantage of your opponent's diverted attention. This can be as simple as a-moving your army in while you storm drop his mineral line, or you can harass in multiple locations simultaneously, or you can take an expansion or perform a violent tech-switch that you'd otherwise be afraid of him scouting. If you do it right, your harass will take most of his attention to defeat, and very little of yours to manage.

3) Never overcommit to harassing. If it takes more than 3-4 units and you getting a little bit of tech you wouldn't otherwise, it's probably an attack and not harassment. This is an important distinction. If I'm harassing somewhere with 2 templars and a warp prism, and my army gets caught in a fight without them, I'll be okay. If I'm attacking with twelve dark templars and the rest of my army gets ambushed by a large force of my opponent's, it'll probably be screwed and the game will turn into a bit of a base trade. When you can harass is different from when you should attack, and understanding the difference can save you awkward losses.

4) Your opponent is rarely prepared. If there are a few spine crawlers shutting down your hellions, or missile turrets shutting down your warp prisms, never fear! Keep your harassment units alive somewhere -- don't throw them away. Continuously scout for more opportunities. Even if your opponent's main is sealed, when he expands you can use those units there instead! Alternately, you can use them to hit the back of his army, or to split his attention somewhere he doesn't have defense.

5) Exploit mobility. This almost goes without saying, but if your units are faster than his, you can do all sorts of cool things: draw out his army, sneak in behind it with a few units, kill all of his drones, and then zip out before he can respond. If your units can move up and down cliffs or fly, be sure you're attacking near a cliff. This will not only increase the amount of damage your units do, it will also fluster your opponent and waste more of his precious apm.