Attacks of Opportunity

The melee combat rules assume that combatants are actively
avoiding attacks. A player doesn’t have to declare anything special
for her character to be on the defensive. Even if a character’s
miniature figure is just standing there on the battle grid, you can be
sure that if some orc with a falchion attacks the character, she is
weaving, dodging, and even threatening the orc with a weapon to
keep the orc a little worried for his own hide.
Sometimes, however, a combatant in a melee lets her guard down.
In this case, combatants near her can take advantage of her lapse in
defense to attack her for free. These free attacks are called attacks of
opportunity (see the diagram on the next page).
Threatened Squares: You threaten all squares into which you
can make a melee attack, even when it is not your action. Generally,
that means everything in all squares adjacent to your space
(including diagonally). An enemy that takes certain actions while in
a threatened square provokes an attack of opportunity from you. If
you’re unarmed, you don’t normally threaten any squares and thus
can’t make attacks of opportunity (but see Unarmed Attacks, page
139).
Reach Weapons: Most creatures of Medium or smaller size have a
reach of only 5 feet. This means that they can make melee attacks
only against creatures up to 5 feet (1 square) away. However, Small
and Medium creatures wielding reach weapons (such as a longspear)
threaten more squares than a typical creature. For instance, a
longspear-wielding human threatens all squares 10 feet (2 squares)
away, even diagonally. (This is an exception to the rule that 2 squares
of diagonal distance is measured as 15 feet.) In addition, most
creatures larger than Medium have a natural reach of 10 feet or
more; see Big and Little Creatures in Combat, page 149.
Provoking an Attack of Opportunity: Two kinds of actions can
provoke attacks of opportunity: moving out of a threatened square
and performing an action within a threatened square.
Moving: Moving out of a threatened square usually provokes an
attack of opportunity from the threatening opponent. There are two
common methods of avoiding such an attack—the 5-foot-step (see
page 144) and the withdraw action (see page 143).
Performing a Distracting Act: Some actions, when performed in a
threatened square, provoke attacks of opportunity as you divert your
attention from the battle. Casting a spell and attacking with a ranged
weapon, for example, are distracting actions. Table 8–2: Actions in
Combat notes many of the actions that provoke attacks of opportunity.
Remember that even actions that normally provoke attacks of
opportunity may have exceptions to this rule. For instance, a character
with the Improved Unarmed Strike feat doesn’t incur an attack
of opportunity for making an unarmed attack.
Making an Attack of Opportunity: An attack of opportunity is
a single melee attack, and you can only make one per round. You
don’t have to make an attack of opportunity if you don’t want to.
An experienced character gets additional regular melee attacks
(by using the full attack action), but at a lower attack bonus. You
make your attack of opportunity, however, at your normal attack
bonus—even if you’ve already attacked in the round.
An attack of opportunity “interrupts” the normal flow of actions
in the round. If an attack of opportunity is provoked, immediately
resolve the attack of opportunity, then continue with the next
character’s turn (or complete the current turn, if the attack of
opportunity was provoked in the midst of a character’s turn).
Combat Reflexes and Additional Attacks of Opportunity: If you have
the Combat Reflexes feat (page 92), you can add your Dexterity
modifier to the number of attacks of opportunity you can make in a
round. This feat does not let you make more than one attack for a
given opportunity, but if the same opponent provokes two attacks of
opportunity from you—such as by moving out of a threatened
square and then casting a spell in a threatened square—you could
make two separate attacks of opportunity (since each one represents
a different opportunity). Moving out of more than one square
threatened by the same opponent in the same round doesn’t count
as more than one opportunity for that opponent. All these attacks
are at your full normal attack bonus.