Shields

A shield affords extra protection at the loss of use of one hand.

Shields serve two useful functions: first, they provide a bonus to AC against the opponent you are facing (and in certain cases multiple opponents).

Second, a shield is the best defense against arrows and other ranged attacks. When a character is aware of an incoming arrow or other ranged weapon (he does not need to actively observe its being fired, merely be aware that the person who is firing it is present), he may spend 2 AP and make a shield check. If successful, the arrow is blocked and negated. A shield check is:

D20 + Dex + Reflex modifier + BDB - facing penalty

Each shield has a different DC to successfully block an arrow, representing the difference between shields with which it is very easy to block (tower shield) and those with which it is nigh impossible to block (buckler).

Any situation which denies you your Dex also denies you the opportunity to defend against projectiles in this manner, though you retain your shield bonus to AC.