Shield Mastery is a Paladin job trait in Final Fantasy XI. It grants a TP bonus when an attack is successfully blocked by a shield. Shield block percentage is determined by the type of shield used. As of the 12/19/2006 version update, a Paladin cannot be interrupted while casting a spell if they successfully block with their shield.
Paladins gain Shield Master I at level 25, granting +1 TP per blocked attack. At level 50, Shield Mastery increase to +2 TP per blocked attack. At level 75, it becomes +3% TP per blocked attack. The one exception to this is that when a blow is blocked and 0 damage is taken, then there is no TP rewarded.