Armor penetration

Armor Penetration (ArPen) is the statistic of a Champion that denies a certain value of the defender's armor. Usually it is placed at 0 (zero); unless an item, that increases the stat, is carried. If the stat isn't a percentage of Armor Pen, the target's armor is directly decreased by the attacker's armor penetration value before the damage reduction is calculated. And Armor Reduction comes before Pen, plus it can reduce enemy's armor below zero. Also there is one considerable difference between flat armor penetration and percentage penetration, which is that the percentage one cannot affect turrets and structures.

Items increasing Armor Penetration

 * +15 Armor Penetration (Unique). +25 Attack Damage, UNIQUE Passive: Reduces ability cooldowns by 10%. 1337 Gold.
 * +20 Armor Penetration (Unique). +30 Attack Damage, +15% Critical Chance, UNIQUE Passive: 15% Cooldown Reduction, Unique Active: You gain +20% movespeed and +50% attack speed for 4 seconds. Attacking enemy units with melee attacks increases the duration by 2 to a maximum of 8 seconds. 60 second cooldown. 2687 Gold.
 * +30 Armor Penetration for 8 sec (Active Unique, cooldown is 40 secs). +55% Attack Speed, UNIQUE Passive: Every fourth attack deals 100 extra Magic Damage. UNIQUE Active: Your attacks cannot be dodged for 5 sec. 2170 Gold.
 * 40% Armor Penetration (Unique). +10 Attack Damage, +40% Attack Speed. 1965 Gold.

Abilities increasing Armor Penetration

 * - - +10/20/30 Armor Penetration

Items increasing Armor Reduction*

 * +20 Armor Reduction to nearby enemy champions (Unique). +20% Attack Speed, UNIQUE Aura: Gives nearby allied Champions 20% Lifesteal, 20% Attack Speed, and 30 Health per 5 seconds regenerated. 2550 Gold.
 * +12 Armor Reduction for 5 sec on each physical attack (maximum 5 stacks) (Unique). +75 Attack Damage. 3065 Gold.

Abilities increasing Armor Reduction*

 * - - +5 Armor Reduction each hit (maximum 10 stacks) for ? seconds.
 * - - +20/25/30/35/40 Armor Reduction for 7 seconds.

*Reduction applies first and can bring you below 0 armor

Updates
Due to recent updates this article might not be up to date. It was last updated during Patch 1.0.0.85. If there is a new version out, please check the data frequently.