Master Yi is probably one of the funnest champions to use in all of League of Legends. He has a fun easy to learn play style that is usually played by building AD. but what many may not know is that he is an extremely viable ap.

A list of terma I use and what they mean
AP: Ability Power
AD: Attack Damage
CC: Crowd Control (Stuns, Snare, Slows ect.)
Magic Pen: Magic Penatration

You minght be asking "Why would I play AP over AD Master Yi. The main reason is because both his Alpha Strike and Meditate scale on AP. This wil give your Q a huge damage output and alot of sustain from his W. We will go into more detail in the Abilities section.

The main focus of your runes is to give you AP, Mana and Mana Regen. Since AP is not given to you every time you level like AD is, these runes become even more important to have AP at level one. Your Marks give you about 5 AP to start with, and the Quints give you around 15. Your Seals are put into getting straight mana as you may need alot of it especially at early points in the game. Your Glyphs are based on giving you both Mana Regen and AP. The Mana Regen works amazingly well as you will probably be staying in lane a long time with Meditate and you will need the mana to hold up that long.

Offence
Summoners Wrath : Improves the cool down effect of your ignite
Mental Force : gives you some extra AP
Sorcery : CDR is Always great on AP's that spam abilities
Arcane Knowledge : Can help out in case enemies begin building magic resist
Demolitionist : Almost all Master Yi Players know that your ult is amazing for pushing towers, with out ad this becomes harder. This masterie will help
Havoc : MORE DAMAGE!
Blast : Gives you additional AP every time you level up.
Archmage : More AP never hurts
Executioner : A must have for any offensive threat, helps you take down those barley surviving enemies.

Defense

None

Utility
Summoners Insight : Reduces you Flash Cooldown
Expanded Mind :Helps you sustain your mana
Improved Recall :Helps you get back to base faster
Meditation :Mana Regen to help you stay in lane.
Transmutation : Spell Vamp to help your abilities.

What Spells should you take? The spells you have can change the game if used right. Below I list all spells in order of usefulness on AP Master Yi.

The Best


Probably the best overall spell in the game. Good for quick get aways when Highlander is down. Or chasing someone down to get in Alpha Strike range.

Good for getting the last bits of HP of someone. Amazing early game for first blood.

The Good

A very underrated spell. Gives you both AP and Attack Speed which is exactly what Master Yi needs.

This spell has so many uses. In can cut down burst damage of the other teams AP. Slow them, and reduce the Armor and Magic Resistance. A very good spell.

The Decent

Never a bad spell to have. Can give just a bit more HP for the team fight or the engagement. But there are better options.

Can be used if your planning on split pushing. You can use attack speed and Highlander to push a tower than Teleport to the fight.

If your a newer player that has trouble conserving your mana this is good for you. If your more experienced than this is unneeded.

Good for newer player who don't have Flash yet. Pretty mush a Highlander

You can use to remove stuns when your being focused in team fights. But Highlander already makes it so you can't be slowed so you should be ok with out this.

DON'T BRING THESE

Only good if your Shen, your not Shen

Only for junglers.

Good for some extra gold or pushing but not to great and there are many other options out there.

This is probably the most important part of this guide, the items.

Boots

Boots are a must have for almost every build.

Great for AP Master Yi. Gives you some extra magic pen.
Mercurys Treads
Good if they have a heavy CC team or if your laning against someone stacking Rabadon's Deathcap.

Good for AD heavy teams, not really reccomended though

CORE ITEMS

Get this item fairly early on. The Auto-attack proc helps so much as you can use it to follow up after an Alpha Strike

The ultimate Item for AP, just gives you so much more damage

It gives AP, CDR and a nice active to help you out

OPTIONAL

Gives you Both AP and AD, some lifesteal and a good passive.

AP and Magic Pen great against tanky teams

If the other is just thorwing Ulti's at you pop this to waste there cooldowns. ALso gives godd amount of AP and sustain with armor.
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Gives you AP, AD and AS everything Master Yi Needs. Also some movement speed.
Guinsoos Rageblade
Good if you 1v1ing some one as you can continue attacking and build up stacks.

Sustain+ Magic Pen

Good if you being focused in teamfights, also gives you a chance to go rambo it to the other team fearlessly.

Good if they have a Karthus or Blitzcrank to pop there abilities.

Good for heavy CC teams.
Rylais Crystal Scepter
Good to catch up and keep up with enemys when your ult is down.

2 of 4 of Master Yi 's skills scale on AP. The other two don't scale.

Master Yi strikes twice every 7th attack
Works better for AD Master Yi but an extra attack always helps.

Master Yi leaps across the battlefield striking up to 4 enemies, dealing 100 / 150 / 200 / 250 / 300 (+100% of ability power) magic damage to each enemy with a 20 / 30 / 40 / 50 / 60 % chance to deal 400 bonus magic damage to minions
This is Master Yi's most powerful skill. It has a 1:1 scale on AP and a Rank 5 base damage of 300. This attack can strike multiple targets and deal around 770 damage in this build. It also has a chance to deal bonus damage to minions which is what makes is suck a powerful move to quickly clear minion waves and push your lane.

Master Yi channels, restoring 28 / 56 / 84 / 112 / 140 (+80% of ability power) health every second for 5 seconds for a total of 140 / 280 / 420 / 560 / 700 (+400% of ability power) health. While channeling, Master Yi gains 100 / 150 / 200 / 250 / 300 armor and magic resistance.
A pretty good ability if your being focused in team fights. It's 4:1 AP ratio gives you major healing. It's main stat is the 300 armor and magic resist it gives you. It's pretty much a Zhonya's Hourglass that heals you. Great to use if your ever in a bad spot or caught out of position. Gives you great sustain in lane also.

Passive: Increases physical damage by 15 / 20 / 25 / 30 / 35.

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.
Good if your pushing towers and the extra AD never hurts. But where focusing on AP so this is pretty much useless.

Increases Master Yi's movement speed by 40% and Attack Speed by 40 / 60 / 80 %, and he becomes immune to all slowing effects for 8 / 10 / 12 seconds. Additionally, killing a champion refreshes all of Master Yi's cooldowns. (Assists reduce half of the base cooldown amount.)
The move that makes Master Yi who he is. The speed is good for both escapes and to chase down enemies. A kill also refreshes your cool downs in order to spam this move.

The Order you level up your skills can have a big effect on the game. For this build you want to start by leveling your Alpha Strike. By level nine have it fully leveled. Level up Highlander when ever it's available. Take two early points in Meditate and have it at rank 5 by level 13. Take an early point in Wuju Style and fully level it at level 18.

Usually playing AP Master Yi you will be in mid-lane. This means you will be against another AP Caster. Use your mobility to avoid there spells and shut down them before they burst you down. Use your Alpha Strike to attack the enemy Champion then follow up with an auto attack proc with your Sheen or Lich Bane. This will put a good amount of damage and scare away the enemy champion. There are two types of lanes you can play with AP Master Yi

OPTION 1 Farmer

This is a typical mid lane. You get as much CS as you can and try not to ush your lane to avoid getting ganked. This is a very safe lane style, its low risk low reward though as you proablly wont be able to get fed early.

OPTION 2: Zoner and Pusher

This is a very aggressive lane style. You push your lane as hard as you can using Alpha Strike followed by a Sheen auto attack. Alpha Strike will occasionally deal extra damage to minions and can take out a whole wave at once. This will cause you opponent to be farming at there own tower. You can then go back and kill the Wraiths camp then return to lane. This can be dangerous and make sure to ward as the jungler may gank you. This is not recommended if you feel that the enemy jungler is camping your lane. You must also always make sure your lane is warded. I will go over ward placement in the warding section.

In team fights you job is to deal as much Damage as possible. Alpha Strike in to the other team. Meditate to protect your self as they mindlessly throw abilities at you. Pop your Highlander then focus there AD Carry , or any other focus your team has decided on. By now you Alpha Strike should be back up so use it again. If you get a kill your Highlander should be back up, and use it again if needed to track down another kill. You should just put as much damage as you can on as many targets as you can.

Remember that when ever your in trouble just Meditate. Also if your Guardian Angel passive is up (assuming you bought one) than save Meditate untill you spawn to heal and run away.

Warding your Lane is Key to playing any position in League of Legends. The morevision you have on the Map the better off your team is.
This picture shows all the places on the map that you should be warding (ASSUMING YOUR ON BLUE TEAM).
CODE
1 :Places you must ward to stay safe
2 :If you are against an aggressive jungler that has moves that can knock you away, I/E Alistar put wards there to get vision. Also to see where the jungler may be coming from.
3 :Helps protect your own jungle and gives you room to farm after your first tower falls.
4 : In case you support is not warding it.
5 : Make sure there is always a ward there after 15 minutes.

These are individual match ups against potential lane match ups. (SECTION NOT COMPLETE)

EASY
A fairly easy matchup. Can dodge all of here skills with your Alpha Strike. Your mobility can be to much for her at times

MEDIUM
You guys both have high auto-attack damage and mobility. Watch out for her Mark of the Assassin and use Alpha Strike on nearby minions when ever she goes into Twilight Shroud

EASY
Your mobility will destroy her, and you easily dodge her Flash Frost. You can Alpha Strike her trough her Crystallize

HARD
She can out range you, stun you and use her sheild to cut down your ablities,. Also watch out for her Summon: Tibbers. You just don't have enough range.

EASY
Has long animations you can easily evade. Use speed to dodge ability's and Minions to block his Sear. Make sure to start with boots.

Hard
If she lands [Petrifying Gaze] on a stun get ready to hear "You have been slain". If she slows you then immediately use you Highlander to get away. Never use you ult offensively as it will cost you. Avoid her [[Miasma] in lane and don't take poison damage.

Medium
You can avoid his Rupture easily. The only hard part is that he can Feral Scream when your using you Meditate to cancel it leaving you vulnerable. His Feast has a small range so you can avoid the damage by staying far away.

Medium
Wont be hard at all, but unless you get really fed before level 6 you both are just goanna heal everything off with Meditate and Maximum Dosage respectively. Just keep a bit of distance and avoid his Infected Bonesaw s

EASY
Just drop a Vision Ward in the middle of the lane and she wont have enough damage to do anything. She runs on the element of surprise. You take that out and she's useless against you.

HARD
Surprisingly hard. Will poke you out of lane and use Reap when ever you use Highlander or Meditate

MEDIUM
He can stop your healing but you can Alpha Strike away from his Chum of the Waters. Probably the best match up for Ap Master Yi

The following users all helped me on my guide either directly or indirectly
jhoijhoi with this guide: http://www.mobafire.com/league-of-legends/build/making-a-guide-101506
Matt with this post: http://www.mobafire.com/league-of-legends/forum/mobafire-support/bbcode-guide-783
Swizzbeat for reminding me about Deathfire Grasp

6/26/12: Guide Released to public. Ahri, Akali, Anivia and Annie added to matchups.
6/27/12: Changed Item Order
6/28/12: Updated Items and Runes, added Rylais Crystal Scepter to build options. Added Brand to matchup section. Added more Depth to team fight section. Fixed coding/grammar errors. Added to Index. Changed skill order.
7/2/12: Added Cassiopeia, Cho'Gath, Dr. Mundo and Evelynn to matchups.
7/5/12: Fiddlesticks added to matchups
7/26/12: IM BACK! Also added Fizz to matchups.