Attack Roll Modifiers

Combat rarely consists of foes standing toe to toe and bashing each other. Movement and position are important; if one archer can fire from behind a tree at an enemy archer out in the open, the one using the tree for cover enjoys an advantage. Similarly, the use of magic or special abilities often creates opportunities that creatures can exploit. If Albanon the wizard turns his ally Keira invisible, she can easily evade her enemies, but if an enemy wizard stuns Keira with a spell, she drops her guard so that enemies can easily gang up on her.
    Temporary advantages and disadvantages in combat are reflected in a set of common attack roll modifiers. An attack roll modifier is a bonus or a penalty that applies to an attack roll in certain circumstances, as determined by the DM.

CircumstanceModifier
Combat advantage against target+2
Attacker is prone-2
Attack is restrained-2
Target has partial cover-2
Target has superior cover-5
Target has partial concealment (melee and ranged only)-2
Target has total concealment (melee and ranged only)-5
Long range (weapon attacks only)-2

Published in Rules Compendium, page(s) 217.