Howdy summoners!

Before anything I'd like to point out that the builds aren't meant for competitive ranked play, in which the items are more situational.

Master Yi requires team play to succeed, otherwise you'll most likely fail utterly, the burst isn't as strong as most AP casters. Master Yi has a high base attack damage and an amazing ultimate to make use of if you're either chasing, in a team fight or counter the other team's stuns, I like to look at yi as a naturally hybrid character, even though I call it an AP build, it'll end up hybrid because after you land the alpha strike you can still help your team with your auto-attacks.

AP Master Yi + Pretty good burst damage, not has good as other casters, but still valuable. + Amazing farm capabilities. + Great harass. + Portable full-hp channeled heal. + Unexpected. - No Stuns or CCs like most casters. - Lacks DPS after Alpha Strike and Lichbane proc. - A single nuke ability. - Harder to play, manage your passive for a greater burst!

SPACE

AD Master Yi + Amazing DPS. + Scary if fed. + His passive will become extremely valuable. - Depends on life steal for survivability. - Very item dependent. - Very Squishy.

I play both, but to be honest I've never used an AP build in a competitive ranked match. It pretty much depends on your team composition, unless you're just messing around and having fun with friends you should first see what your team needs. You need a team who's capable of adapting to Master Yi's playstyle as it is unusual. Try it with a premade.

• Greater Mark of Magic Penetration - Provides magic penetration which pierces through your enemies' magic resistance.
• Greater Seal of Replenishment - Provides mana regeneration, mainly useful early to mid game so you don't need to go back to base to replenish your mana.
• Glyph of Cooldown Reduction - Provides cooldown reduction, important as Master Yi mostly relies on a single ability with an averagely high cooldown, mainly on low levels.
• Greater Quintessence of Magic Penetration - Extra magic penetration.

Those aren't the only viable options for Master Yi, but are the ones that seemed to fit my playstyle the most. You can mess around with your own runes.

I'll only point out special cases which might cause confusion:

Why Awareness over Expanded Mind and Good Hands over Perseverance?

Despite the look, neither Expanded Mind nor Perseverance are very significant at all, I personally find Awareness and Good Hands to be more useful late in the match than their counter-parts.

For further detailed information please take a look at this jhoijhoi's guide.

• Flash - Will both work as escaping and chasing mechanism. Make good use of it and you wont play without it ever again :)
• Ghost - Synergizes with Highlander
  
• Ignite - To assure a kill and reduce healing done to the affected champion.
• Exhaust - Useful when chasing and to disable enemy's combat capabilities.
• Cleanse - An extra anti-CC, in addition to Highlander.
• Teleport - For map control, in most cases the tank will take it instead.
• Smite - ALWAYS use it if you're jungling.
  
  --- Less Viable: ---
  
• Heal - Viable in some cases, mostly to fool your enemy into chasing you to assure a kill.
• Clairvoyance - Leave it for your team's support.
• Revive - Usually underestimated, nevertheless I do not recommend it.
• Clarity - Overrated spell, not worth it.
• Rally - Can be useful in some cases, but not recommended.
• Fortify - Leave it for the tank / support.

Alpha Strike - Master Yi leaps across the battlefield striking up to 4 enemies, dealing 100/150/200/250/300 (+1) magic damage to each enemy with a 20/30/40/50/60% chance to deal 400 bonus magic damage to minions.

Your primary burst ability. To harass you have to make a smart use of it, do not directly target enemy champions with it unless you're sure you wont be outplayed, use the minion wave, it'll strike nearby enemies, so if you use it in a minion close to you it'll eventually strike the enemy champion(s) and still keep you in a safe distance.

This ability's AP ratio is 1:1, that means, for each point in AP you have, it'll increase its damage by +1. It is pretty awesome.




Meditate - Master Yi channels, restoring 140/280/420/560/700 (+1.65) health over 5 seconds. While channeling, Master Yi gains 100/150/200/250/300 armor and magic resistance.

This one will heal a ton once you get some AP, it also provides with insane amounts of resistances while channeling, don't be afraid of using it in the middle of a fight if you can't sustain the damage.




Wuju Style - 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.

This is the source of your high base damage, use it whenever you're able to, this and Highlander will still hurt even though you have no attack damage items.




Highlander - Increases Master Yi's movement speed by 40% and Attack Speed by 40/60/80%, and he becomes immune to all slowing effects for 6/9/12 seconds. Additionally, killing a champion refreshes all of Master Yi's cooldowns. (Assists reduce half of the base cooldown amount.)

Your ultimate. Use it wisely, disrupts CCs and increases both your attack and movement speeds by a very significant percent. Don't you dare use it to get to a certain place quickly, and don't justify it with the EXP loss you'd get if you didn't use it.




Double Strike (Passive) - Master Yi strikes twice every 7th attack.

Your passive. A very valuable stack to manage, use it together with Wuju Style for an optimal burst, early game use it with the sheen proc. Its a significant addition to your AP burst.



Optimal burst ability sequence:

-> -> -> -> Right click to auto attack, just to make sure.

Viable items:

*Note: The items on the list have no particular order.*

Sorcerer's Shoes - Magic Penetration, to pierce through enemies' magic resistance.
Ionian Boots of Lucidity - More cooldown reduction is always welcome.
Mercury's Treads - Adds a little bit of magic resistance along with CC reduction on you, I usually pick those if the enemy item has strong CC champions such as Ashe.
Boots of Swiftness - Extra movement speed, I'm not really an apologist of this item.

Rabadon's Deathcap - Pure AP, increases the efficiency of all other AP items by 30%.
Rylai's Crystal Scepter - I chose this item mostly because of the slow (your team will be grateful), the extra hp comes in handy too.
Lich Bane - Highly recommended item, it'll deal your total AP in damage on the next attack once it procs, it'll improve your burst a lot.
Will of the Ancients - Provides you and your team some spell vamp, which is the life steal of the abilities. It'll heal you for a percent of the damage dealt.
Void Staff - In case the enemy team is stacking a lot of magic resistance. Just a personal thought: Replace Morello's with this one if you really need it.
Rod of Ages - For more survivability, buy in a early phase of the game so it'll be effective mid game.
Morello's Evil Tome - AP, mana regen, cooldown reduction.
Deathfire Grasp - Very powerful item, buy it if your foes are stacking pure HP.
Zhonya's Hourglass - To be honest I don't like this item, but it provides you with armor and large amounts of AP.
Nashor's Tooth - Works surprisingly well with Master Yi, massive cooldown reduction and attack speed in addition to yi's high base damage.
Abyssal Mask - Magic Resistance for more survivability and a magic resistance reduction aura that affects nearby enemy champions.


Sample Builds:

My Standard:


Survivability:
or

Strong Burst:

As an AP Master Yi you SHOULD NEVER initiate. Stay back until the tank initiates and only then you can use your combo to get into the fight.

In the laning phase you want to farm as much as possible. Do last hits. You've got a pretty strong zone control, use Alpha Strike in the minion wave to harass the enemy champions from a safe distance and at the same time get some free gold. Don't overextend too much, yi is item dependent, dying will not help at all, better play safe and farm, farm, farm! But caution, always be aware of Yi's squishiness, if you're ganked you can be taken out easily. Try to cooperate with your lane partner and try to make your lane partner cooperate with you: AP Yi is very dependent on team play.

In teamfights always target the squishy champions first, ignore the tanks, with your burst you'll be a big, fast, furious threat. Keep in mind you should never wander around the map without your team's support.

Also, do not think that being Master Yi frees you from buying wards, map awareness is extremely important, mainly in competitive environment, everyone should buy at least 2 wards at every recall.

AP Yi is very fun to play, and you basically make your enemies' harassment attempts completely pointless because you have a heal that replenishes your hp in a full channel, at a low mana cost. I usually have no issues with mana, simply because of the fact I do not spam Q the whole game without thinking what I'm doing.

If you have Yi, give it a try, its definitely worth it.

A good gameplay commentary I found on youtube:
Part 1: Play Video
Part 2: Play Video

19 July, 2011: Guide Released.

I'd highly appreciate user feedback, please tell me if I forgot something and of course I'm open for suggestions. Help me update the guide.

Thank you very much and please vote wisely, if you down vote please explain me why so I can improve it, don't troll over the effort I put in the guide.