SRD:Shield Bash

Shield Bash Attacks: You can bash an opponent with a shield, using it as an off-hand weapon. See Table: Weapons for the damage dealt by a shield bash. Used this way, a shield is a martial bludgeoning weapon. For the purpose of penalties on attack rolls, treat a shield as a one-handed weapon. If you use your shield as a weapon, you lose its AC bonus until your next action (usually until the next round). An enhancement bonus on a shield does not improve the effectiveness of a shield bash made with it, but the shield can be made into a magic weapon in its own right.

System Reference Document