|d20 Modern System Reference Document|
ATTACKS OF OPPORTUNITY
The melee combat rules assume that combatants are actively avoiding attacks. A player doesn’t have to declare anything special for his or her character to be on the defensive. Sometimes, however, a combatant in a melee lets his or her guard down, and doesn’t maintain a defensive posture as usual. In this case, combatants near him or her can take advantage of this lapse in defense to attack for free. These attacks are called attacks of opportunity.
A character can use a melee weapon to make attacks of opportunity whenever the conditions for such an attack are met (see Provoking an Attack of Opportunity, below). In addition, a character can make attacks of opportunity with unarmed attacks if the character’s unarmed attacks count as armed (see “Armed” Unarmed Attacks).
A character threatens the squares into which he or she can make a melee attack, even when it is not the character’s action. Generally, that’s all squares adjacent to the character’s position. An enemy that takes certain actions while in a threatened square provokes an attack of opportunity from the character. A character can only make attacks of opportunity with melee weapons, never with ranged weapons.
Provoking an Attack of Opportunity
Two actions can provoke attacks of opportunity: moving out of a threatened square, and performing an action within a threatened square that distracts from defending and lets the character’s guard down.
Moving out of a Threatened Square: When a character moves out of a threatened square, that character generally provokes an attack of opportunity. There are two important exceptions, however. A character doesn’t provoke an attack of opportunity if all he or she moves is a 5-foot step, or if the character withdraws.
If the character doesn’t start in a threatened square, but moves into one, the character has to stop there, or else he or she provokes an attack of opportunity as he or she leaves that square.
Performing an Action that Distracts the Character: Some actions, when performed in a threatened square, provoke attacks of opportunity because they make a character divert his or her attention from the fight at hand. Using a ranged weapon, in particular, provokes attacks of opportunity. Table: Actions in Combat notes many additional actions that provoke attacks of opportunity.
Making an Attack of Opportunity
An attack of opportunity is a single melee attack, and a character can only make one per round. A character does not have to make an attack of opportunity if he or she doesn’t want to.
An experienced character gets additional regular melee attacks (by using the full attack action), but at a lower attack bonus. A character makes his or her attack of opportunity, however, at his or her normal attack bonus—even if the character has already attacked in this round.