1. Intro
Hey guys I'm really bored, so i felt like making a Master Yi guide, and showing how i approach him as a champion. I've been reading guides for a long time so i felt like giving something back the community, even if it's a Yi guide that we a couple thousand of. I'm also using this as practice for English writing.

2. Story
Keeper of the ancient art of Wuju, Master Yi is a mystical warrior from the isle of Ionia. His village was steeped in Wuju, and his kinfolk were some of the few to carry the tradition into the modern day. That is, until the armies of Noxus came to Ionia, ravaging the land. Master Yi went to fight the craven enemy, bringing his village honor through his unparalleled mastery of the blade. However, in his absence, the armies of Noxus closed in and tested the latest invention of the chemist now known as Singed. What was done to his village is something of which Master Yi will not speak. However, he has brought his mastery of the blade to the League of Legends in order to avenge his kinfolk and end the predations of the Noxus once and for all.Master Yi brings the art of the Wuju style in a way that makes his enemies falter. He is capable of fighting with lightning speed, as well as finding purchase behind the defenses of an enemy's armor. He can call on his inner fortitude, regenerating at an accelerated rate. Calling upon ancient forces, Master Yi can fold space, appearing behind enemy after enemy. He is even able to reject magic, rendering it useless against him.The art of Wuju lives on in Master Yi, and is plunged through the hearts of his foes.

3. Skills

Passive - Double Strike - Master Yi strikes twice every 7th attack.
- A really good passive, synergizes greatly with Yi's natrually high attack speed

Q - Alpha Strike - Master Yi leaps across the battlefield striking up to 4 enemies, dealing magic damage to each enemy with a good chance to deal 400 bonus magic damage to minions.
- Your Farming, Chasing, Initiating, Wallhax skill. Use this often to catch up to low health enemies and deliver the death blow. Has invincibillity frames as Yi travels, and is great lategame for tower diving. Lategame this can pretty much one-shot 4 minions at once. With AP builds, Alpha Strike will do a massive amount of damage.

W - Medidtate - Master Yi channels, restoring health over 5 seconds. While channeling, Master Yi gains armor and magic resistance.
- Lategame this heals ~60% of your hp. Great for recovering in between teamfights. Meditate also gives a ****load of armor and magic resistance once it's maxed, so it can be used as a defense against "for sure hit skills" such as Karthus' Requeim.

E - Wuju Style - Passive: Increases physical damage. (Active): Master Yi can activate this ability to double its effectiveness for 10 seconds, but this removes the passive bonus until Wuju Style becomes available again.
-More attack damage. You want to keep this off cooldown unless youre fighting champs/turrets/baron/dragon.

R - Highlander - For several seconds, increases Master Yi's speed of movement and attack by a large percentage, plus makes him immune to all slowing effects. Additionally, killing a champion refreshes all of Master Yi's cooldowns, and scoring an assist reduce the cooldowns by half.
- This is what makes Master Yi Master Yi. Pop this along with wuju, and youll pretty much rip anyones face off. This skill is also widely used as an escape mechanism.

4. Masteries

Classic Yi - 21/0/9
AP Yi - 9/0/21
Tank Yi (Build Coming Soon)- 0/21/9

Runes

Armor Penetration Red + Quints- Increase damage.
Attack Speed Yellow - Increase damage.
Cooldown Reduction Blue - Increases spamabillity.

5. Summoner Spells

Reccomended:
Ghost - Perhaps the most versatile summoner spell. Paired with Cleanse and Highlander, there's really nothing you can't get away from.
Exhaust - Great for ganking early game. I often take this over Cleanse.
Cleanse - Removes all CC. Synergizes well with Ghost and Highlander which make for a excellent escape mechanism.

4. "Classic" Yi
Pros:
High Damage Capabillity
Backdoors like a boss
Great Carry
Unstoppable when fed
Cons:
Squishy
Focused, CCed, Taunted
Mana hungry early game if you spam Alpha
Skill Build:
Q
E
E
Q
E
R
Q
E
Q
E
R
Q
W
W
W
R
W
W
Item Build:

Core Items - Boots, Infinity Edge, Phantom Dancers (2x)
Boots of Speed + 3 Health Potions
Boots - Berserker's Greaves - Balanced, Ninja Tabi - AD Heavy, Mercury's - CC or AP Heavy
B.F. Sword
Cloak of Agillity
Pickaxe
Combine B.F., CoA, and Pickaxe to make Infinity Edge
Zeal
Zeal
Phantom Dancer
Phantom Dancer
If the game is still going on, get two situational items of your choice. I usually get a Bloodthirster and a survivabillity item.
Reasoning: Your abillity to pump out damage will be unmatched. Youll crit 80% of the time, usually around ~700 damage with more than 2.0 attack speed. You'll easily 2-4 shot squishy champions, and if you got a Bloodthirster than the lifesteal will pretty much let you destroy 1v1 and heal up insanely quick inbetween fights (lifesteal is kind of useless in teamfights since the enemy team will focus you like crazy and it wont really make a difference).
5. Abillity Power Yi
Pros:
Epic damage when Alpha'ing
Medidate will heal a crazy amount of health
Less Squishy then Classic Yi
Lower CD (45 sec Highlander, 6 sec Alpha)
Can nuke huge amounts of damage with Alpha, and do a DECENT amount of damage with auto-attacks (however, not even close to lategame Classic Yi)
Fun to play
Cons:
Decreases the effectiveness of Wuju and Highlander
Terrible damage early game
Heavily reliant on Alpha
They stack Magic Resist -> You shud probably get Void Staff, if not, youre ****ed
Skill Build:
Q
W
Q
E
Q
R
Q
W
Q
E
R
W
E
W
E
R
W
E
Item Build:

Core Items: Boots, Lich Bane, Mejai's, Rabadon's Deathcap, Nashor's Tooth
Saphhire Crystal + HP Pots
Situational Boots
Sheen
Mejai's Soulstealer (Don't hate on snowball items. Remember, you can stack with assists)
Fiendish Codex
Nashor's Tooth
Rabadon's Deathcap (at this point your Alpha will be doing a good amount of damage)
Lich Bane
Zhonya's Hourglass, or if theyre stacking Magic resist get a Void Staff
Full Build: Boots, Lich Bane, Mejai's, Rabadon's Deathcap, Nashor's Tooth, Zhonya's Hourglass or Void Staff

Reasoning: With this build you should have ~530 AP w/o Mejai's stacks. With full stacks it comes to a total 741 AP. That means youll be nuking close to 1000 dmg to each of your four Alpha victims. In addition to this, Alpha Strike is on a 6 second cooldown. Mejai's is a great choice considering the amount of AP it gives you in relation to it's cost. At 5 stacks, Mejai's is an item that is already worth to keep (unless it's late game and you have enough gold of a better item). It is relatively easy to achieve stacks. A few ganks, teamfights, and skirmishes, youll have your 20 stacks. If you absolutely hate Mejai's then a good replacement would be Hextech Gunblade, seeing it helps your "hybridabillity".

6.1 Strategy - Backdooring

Pros:
Wins games.
Cons:
Enemy team will rage and hate on you like no tommorow. (hmm.. this can be a pro)

In my op, Yi is the best backdoorer in LoL. Period. His speed and abillity to pump out pure damage without doing anything else is the reason why.

Essentially, backdooring is when the enemy team is distracted / busy performing a certain task, and you, the BDer, will be running up one of their lanes and killing their turrets. For example, the entire enemy team is pushing mid. You tell your teammates to turret hug while you run up bot lane and take out their turrets and inhibtor, perhaps even their nexus.

Obviously, if the other team has half a brain, they won't just let you backdoor. The ones with global tele skills (Grand Skyfall, Destiny, Teleport Summoner Spell) will most likely come to take you down, or they might cast Fortify and their entire team aborts the push and calls back to defend the turrets. In order to BD you must communicate extremely well with your team.

If you wish to backdoor, you and your team must follow these simple rules.
Good communication.
DO NOT BD if you know your team will be annihilated without you while they're defending.
DO NOT BD if they are MIA.
When one of their team goes missing your team must immediately call MIAs. Therefore you can judge wether you'll be able to break the turret down or if you'll have to abort.
DO NOT hesitate on using your ulti.
6.2 Strategy - Gank / Chase / Escape

All of the above three topics are relatively easy with Yi. To gank , simply initiate with Alpha, pop Highlander and Wuju, then just autoattack the enemy. If they are about to escape, Alpha to get back where you want to be, right beside them laying waste with your autoattacks. If you somehow get in a sticky situation, NEVER HESISTATE TO USE YOUR ULT. Just pop it and gtfo of there. With Ghost + Cleanse you can pretty much get away from anything, since their CC will be useless and you will be running faster than Usain Bolt.

6.3 Strategy - Advanced Alpha Strike Tactics
Under normal circumstances, once Alpha Strike has been casted on an enemy, it will follow and hit the enemy no matter what. There are a few exceptions to this. Items or skills such as Sivir's Spell Shield or Noctune's Shroud of Darkness and Banshee's Veil's active can nullify Alpha's effects. Another exception is if you Alpha'd to an enemy and while you were flying in the air your target has died, you'll return back to your original position. You can take advantage of this while turret hugging. While the original target of Alpha is dead, Alpha still hits another 3 random enemies, and youll return to your orignal position. So it's basically like a ranged attack. This will be one of your main turret hugging techniques. However, this skill takes time to master. Back on topic, as I have said, there are few exceptions, which means that Alpha is an extremely versatile chasing tool. Alpha can also help against wallhackers such as Tryndamere's Spinning Slash. As I ahve said, once Alpha Strike has been casted on an enemy, it will follow and hit the enemy no matter what. Which means if I cast Alpha on Tryn and he Spinning Slashes through a wall right after, I'll follow him through the wall and still hit him.

Enemies that can put up inpenetrable objects such as Anivia, Jarvan, or Trundle can be dealt with using Alpha. All of these temporary walls can be Alpha'd through. I can't count how many times I've alpha'd into a Cataclysm arena to help out a teammate getting bashed by Jarvan.

Tower diving low health enemies with Alpha Strike is awesome, since you'll have invincibillity frames as youre flying through the air towards your enemy (do not try this at early levels, unless you are sure you can survive the turret shots).