See the D&D FAQ
This is discussed very well in the D&D FAQ:
My group’s having some difficulty with the Expertise
feat. What happens when you use Expertise along with
Two-Weapon Fighting? Do you get the Armor Class
bonus from Expertise once for each weapon? Once for
each attack? What if you use the total defense option or
the fight defensively option? (Or is Expertise just
another way of fighting defensively?) If you have five
or more ranks in the Tumble skill, you get a bonus to
defensive fighting or total defense. How does that work
with Expertise?
Any attack bonus you devote to defense with the
Expertise feat (up to your base attack bonus or +5,
whichever is lower) applies as a penalty to all the attacks
you make until it’s your turn again, including attacks of
opportunity. You get the defense bonus only once, no
matter how many attacks you make or how many
weapons you use. (It, too, remains until it’s your turn
again.) For example, a 9th-level fighter with a base attack
bonus of +9/+4 has Expertise, Two-Weapon Fighting,
and Ambidexterity. If the character uses a light weapon
in her off hand, she’ll get 3 attacks, each at a .2 penalty:
+7/+2 with her primary hand and an extra attack with
her off hand at +7 . If the character claims a +2 bonus to
Armor Class from Expertise, her attack bonuses become
+5/+0 and +5.
As pointed out before, you can’t use Expertise and total
defense at the same time. (You can use Expertise only as
part of the full attack or attack actions.)
You can fight defensively only when you use the full
attack action or attack action (see page 124 of the Player’s
Handbook), but if you do, you can use Expertise along
with it. (Because fighting defensively is mentioned in the
Expertise feat description on page 82 of the Player’s
Handbook, some people think Expertise is just a better
way of fighting defensively, but that’s not true.) When
you use Expertise and defensive fighting together, you
simply add up all the Armor Class bonuses and attack
penalties. Note that both Expertise and defensive fighting
provide dodge bonuses to Armor Class. Dodge bonuses,
unlike most other bonuses in the game, stack. (See page
119 in the Player’s Handbook.) If the character in the
previous example decided to fight defensively, she would
add another +2 bonus to Armor Class (for a total of +4)
and an extra .4 attack penalty, which would reduce her
attacks to +1/.4 and +1. The combination of Expertise
and defensive fighting isn’t very efficient.
If the character in the example also had 5 or more ranks
in the Tumble skill, her dodge bonus from fighting
defensively would be +3 instead of +2, and her total
Armor Class bonus would be +5.
Since Expertise is not the same as fighting defensively,
your Tumble ranks don’t help you when you use
Expertise without also fighting defensively.
with an attack that can deal damage.) When you’re
overrunning, you’re making a trip as part of the move
portion of a charge, not as a melee attack. You can,
however, end your charge and use your normal attack
against an opponent you’ve knocked down during an
overrun.
