Hide (Dex; Armor Check Penalty)
Your Hide check is opposed by the Spot check of anyone who might see you. You can move up to one-half your normal speed and hide at no penalty. When moving at a speed greater than one-half but less than your normal speed, you take a -5 penalty. It's practically impossible (-20 penalty) to hide while attacking, running or charging.
A creature larger or smaller than Medium takes a size bonus or penalty on Hide checks depending on its size category: Fine +16, Diminutive +12, Tiny +8, Small +4, Large -4, Huge -8, Gargantuan -12, Colossal -16.
You need cover or concealment in order to attempt a Hide check. Total cover or total concealment usually (but not always; see Special, below) obviates the need for a Hide check, since nothing can see you anyway.
If people are observing you, even casually, you can't hide. You can run around a corner or behind cover so that you're out of sight and then hide, but the others then know at least where you went.
If your observers are momentarily distracted (such as by a Bluff check; see below), though, you can attempt to hide. While the others turn their attention from you, you can attempt a Hide check if you can get to a hiding place of some kind. (As a general guideline, the hiding place has to be within 1 foot per rank you have in Hide.) This check, however, is made at a -10 penalty because you have to move fast.
Sniping: If you've already successfully hidden at least 10 feet from your target, you can make one ranged attack, then immediately hide again. You take a -20 penalty on your Hide check to conceal yourself after the shot.
Creating a Diversion to Hide: You can use Bluff to help you hide. A successful Bluff check can give you the momentary diversion you need to attempt a Hide check while people are aware of you.
Action: Usually none. Normally, you make a Hide check as part of movement, so it doesn't take a separate action. However, hiding immediately after a ranged attack (see Sniping, above) is a move action.
Special: If you are invisible, you gain a +40 bonus on Hide checks if you are immobile, or a +20 bonus on Hide checks if you're moving.
If you have the Stealthy feat, you get a +2 bonus on Hide checks.
A 13th-level ranger can attempt a Hide check in any sort of natural terrain, even if it doesn't grant cover or concealment. A 17thlevel ranger can do this even while being observed.
The character can hide others as well as hiding him or herself.
Hide Another: By accepting a -30 penalty on his or her Hide check, a character can hide another adjacent creature whose size is no more than one category larger than the character's own. Modifiers to the check for the size of the creature still apply, as do all other penalties, including those for moving faster than half speed. Likewise, a character can only hide another creature when it is not under direct observation by a third party. The creature the character hides remains hidden until it is spotted or it takes some other action that breaks its concealment, as normal.
Source: SRD 3.5 Combined Skills