So Ive been playing the game for awhile now, but I still havent grasped the concept of which champs armor pen runes/masteries is best on... Can someone clarify this for me?

AD champs are best with armor pen: such as junglers and solo tops. Helps early game. A few adc's can work with armor pen.

Generally melee champs that don't scale super well with AD would rather have ArmPen.

IE: Jarvan IV

Hmm, clears up alittle... but not alot. I just dont understand why armor pen is good on somes and not others, whereas those others you would run AD instead.

Makes sense. How do I know which champs scale well with AD or not?

http://leagueoflegends.wikia.com/wik...f_Legends_Wiki

This website contains information on every champion. Included in this information is info on their abilities, which includes what they scale to and at what rate.

Ayeayeaye... our communities a little out of shape I see. I'll try and explain to you why armor penetration became popular and why its better than AD, and why you'd take one over the other. I went ahead and labeled the parts of this post because it ended up being stupidly long.

For the record... its ArPen, or Arpen, not armpen, that sounds stupid...


Flat Armor Penetration
Armor Penetration is up first... there is one sole reason why anyone even considered taking Arpen over Flat or Scaling AD and that is... Consistent damage scaling. If a champion deals all physical damage (No bonus magic damage) and dosen't waste the bonus damage from Arpen, they will consistenly deal more damage than what they would've done if they had Flat AD runes early game, and Scaling AD runes lategame.

Percent Armor Penetration
Now we're taking Flat Arpen, not percent Arpen. Percent Arpen deals more damage the more armor an enemy has and is therefore good for tank-busting, but little else. If it is stacked, percent Arpen masteries, Last whisper, and a percent armor penetration ability, can be stacked to reduce armor by 60-75% depending on the ability used and can really put the hurt on enemies who are trying to use armor to counter your damage.

Comparison
Now why did I mention the difference between the two? Because if you take Flat Penetration along side Percent Penetration, the total amount or armor you ignore is reduced by Flat pen. first, then percent. So you will waste your own resources against armor because if an enemy has 66 armor, you build 15 Arpen from runes (Runes only have flat pen available), 10% arpen (From Weapon Expertise), and the Black Cleaver, you will only take out 62(ish) damage, instead of 66 you could've shreded if you had put 3 points into Sunder, Sure thats not a fair matchup, but at least you see the difference between flat and percent Arpen.

So with that in mind, don't actually take Flat Arpen runes with champions like J4, who have innate percent armor shred in their ability kits. Some other champions that have percent armor shred are Urgot, Wukong, Shyvana, among others.

(While we're on the subject, J4 is classed as a Bruiser, which in most cases, means a cross between a Mage and a DPS, he deals his damage through a combination of his abilities and auto attacks)

Generally on these champions, avoid Flat Arpen because it won't give you the benefits that Flat AD or other offensive stats (Attack Speed, Crit Rate, etc) could have. ESPECIALLY if your champion deals any sort of magic damage, then it's a complete waste to take armor penetration whenever you deal magic damage. However, Flat armor penetration is great for amplification against squishier targets who have less than 100 armor (If they have more than 100, last whisper will deal more damage to your target). Just be aware that it can conflict with other armor reduction abilities.

Attack Damage

Now lets finally get on to AD runes. Attack damage runes give good early or late game damage (Flat or scaling respectively) for champions that scales well with AD, but less than if a champion can make good use of armor penetration.

So in the end, most champions who deal all physical damage should take Armor penetration, but if they have a percent armor penetration ability, they generally would be better off with AD or even AS. Read up on the champion you want to play, and see what other people say regarding that particular champion.

GLHF and ask more questions if you have them, I'd like to dig up an old Armor penetration vs Attack damage guide I like, but I can't seem to find it... and you already have a link to the Wiki so I'll leave you at that.

While the previous post is a pretty good explanation on the differences, the tips offered at the end are a little off in my opinion.

Armor penetration will always be better late game than attack damage. All champions have a certain amount of armor even if they don't build any. Reducing that armor by any amount at the end of the game will result in you doing more damage than simply having a few points of extra attack damage (except possibly with the case of a champion like Riven and the way her passive works).

However, early game, attack damage is better. It's not necessarily better because you'll do more damage to the enemy champions in every case (in some cases you will, some you won't, depending on how much and how fast they build armor). Attack damage is better early game because it makes last hitting easier. The first two waves of minions have ZERO armor. They get some with the 3rd wave I believe, and it increases as the game drags on, but nonetheless, it takes a while for the minions to get enough armor where armor pen would result in more damage dealt to them than attack damage runes.

And why is this important?

Because you never know how the enemy champion is going to build. Maybe they'll stack a ton of armor, maybe they'll stack a ton of damage and no armor. But if you use AD runes and last hit effectively, you'll have plenty of gold to build whatever items you need. Maybe you need more armor, maybe you need a Last Whisper for some armor pen, but whatever the case, it's better, in my opinion, to cover as many stats as you can in game with items, because this allows the most amount of flexibility. Using AD runes to ensure maximum last-hitting power puts you in the best position to have the gold you need to custom tailor your item build to your enemy.

Thanks for cutting me some slack, it was like 3 in morning I decided to finish that. But yes, Pogo's right here, taking Flat AD in terms of a ADC is very good for last hitting. However if you want to play other champions who...

A, have a strong lasthitting ability
B, have very high base damage
C, are in the jungle early game

You want to look at their kits and make sure that the runes you take synergize with their other abilities to make the most of your runes early game, or lategame, if you're into that sort of thing.

Remember also, to decide whether or not your champion shines late game or early game, lets look at AD Kog'maw and Urgot for a sec.

Both have extremely long range abilities, high physical damage, an armor reduction ability, and a kite-friendly slow.

Urgot will obilerate enemies in the early game, his laning used to be (And still is) one of the most unshakable in the game simply because he could hit enemies from across the map with Acid Hunters. His range has been nerfed, but the point is, Urgot is a champion who needs help in the Lategame, because his early game is devastating by itself without runes.

Kog'maw on the other hand is a carry, and tends to have low(ish) early game damage and depends on his team for protection, however, once he gets fed he becomes a devastating killing machine, being able to raze even the toughest champions in the game very quickly. So you would want to make sure his early game is set up to get him into his lategame so he can really shine. Some defensive Runes and items early can help him out with this.

So bear in mind how you play your champion, and what their strengths and weaknesses are, and choose your own runes accordingly.

On a side note, you can also just pull up a guide on a champion you like and figure out which runes people seem to like, you don't even have to read the entire thing!

GLHF

The way armor and resist works makes Penetration better. Each point of armor and resist makes your character take 1% more of their health to kill. 100 of each would make you twice as much damage before dying, 200 = triple damage, etc.

An enemy has 1,000 hp and 100 armor and resist; effective health 2000. If you have 10 penetration, then for the purpose of facing you, they actually have only 90 armor; effective health 1900. The more health an enemy has, the stronger the effect of penetration. 2000 hp = 4000 effective = 200 hp knocked off by 10 pen. 3000 hp (again, this is with 100 armor+resist) = 6000 effective health minus 300 hp for every 10 pen.

Last Whisper and Void Staff each knock off 40% of the target's armor and resist respectively, which is why a lot of people say you should just buy one of those and focus the rest of your time on damage. Stacked percentage reductions become less effective (10% pen masteries + 40% Void Staff = 46% magic pen). Flat reductions stack without becoming less effective, though.

Personally I think penetration beats the heck out of damage.