Armor Class (AC). AC is a number that represents how well a creature avoids being wounded in battle. You successfully hit your foe if your attack roll, with all modifiers, equals or exceeds its AC. 

With the optional rule on facing, AC does not benefit from shields in certain situations.

Cover  

Spells

Potions

AC =

No Armor, Natural Armor, Manufactured Armor, or Unarmored Defense

+ 

Shield

+ 

Dexterity Modifier

+ 

Magic Items

Temporary

Effects

+ 

Armor Table

Armor FAQ

 Certain armor limits or prevents application of a Dexterity modifier to AC.