One of the most common attack roll modifiers is combat advantage, which represents a situation in which a target can’t give full attention to defense. The target is pressed by multiple enemies at the same time, stunned, distracted, or otherwise caught off guard. Combat advantage has two rules.
+2 Bonus to Attack Rolls: A creature gains a +2 bonus to attack rolls against a target granting combat advantage to it.
Able to See Target: A creature must be able to see a target to gain combat advantage against it. This rule means a blinded creature cannot have combat advantage against anyone.
Once per encounter, a creature can try to gain combat advantage against a target by making a Bluff check.
Combat advantage is relative. In any given pair of combatants, either, both, or neither might have combat advantage against the other. It’s possible for a single creature to be adjacent to one enemy that has combat advantage against it and a second enemy that does not.
Published in Player's Handbook, page(s) 279, Monster Manual, page(s) 280, Rules Compendium, page(s) 217.