Immune: undead, constructs, elementals
An exhausted character moves at half speed, cannot run or charge, and takes a –6 penalty to Strength and Dexterity. After 1 hour of complete rest, an exhausted character becomes fatigued. A fatigued character becomes exhausted by doing something else that would normally cause fatigue.
Marius, Scholai Armsmaster says: Though the -6 to strength and dexterity can be devastating, it is the half speed and inability to charge that makes exhaustion truly devastating. Combined with difficult terrain and hit-and-run tactics, there are few things easier than outrunning an exhausted opponent. Necromancers excel at exhausting their enemies with spells like Ray of Exhaustion (sor/wiz3) and Waves of Exhaustion (sor/wiz7). As an area of effect spell with no saving throw, the latter spell can easily cripple an entire squad of enemies.