I'm not diamond, platinum, challenger or anything, I'm only gold.
I play Lee Sin, he's my favorite, my best, and I like to think I'm good at him. This is a guide if you're wanting to play but don't know where to start.

Before I go on, if you're jungle, take Exhaust/ Flash and Smite, same build.

IGN: ShekalAchaea - Feel free to add me if you wish to play or ask questions :)

Why Lee Sin? He's just a blind bugger who can still be affected by Teemo's Blinding Dart.

Lee Sin, as you know, is a ranged, melee, tanky DPS, assassin, mage, tank, support and jungler, all wrapped up in one defective man.

PROS:

• Very high damage early game.
• Good CC, always useful in teamfights.
• Strong ganks, especially at level 2.
• Fun to play.

CONS:

• Falls off late game if built for more damage than tank.
• Somewhat difficult to learn and master.

Lee Sin can make amazing plays that always look impressive, his kit gives him great mobility and ganks. However he falls off with damage later in the game compared to most other bruisers.

Utilizing passive. Flurry
In exchanges, this is most useful, especially in top lane, be sure to use your passive any chance you get, free attack speed is good!
IE: Attacks would go Q > AA > AA > Q > AA > AA > E > AA > AA > E, etc Play Video
Q R Q combo.
Using this combo, especially when built with higher damage and Last Whisper for armor penetration, can cause very high burst damage, as well as singling you and your target out of a teamfight if kicked over a wall, to focus him down.
Play Video
Ward jumping.
Carrying wards is always a good idea, which is why Wriggle's Lantern or Ruby Sightstone were made for Lee Sin, a free ward to jump to, damage, and lifesteal, with some extra armor. Lee's W can jump to wards, and other friendly objects.
Objects include: Minions, and friendly objects, such as Heimerdinger's H-28G Evolution Turrets, Zyra's plants, Shaco's Jack In The Box and much more Play Video
Delivering kills.
Delivering kills is what a jungler and good bruiser does, it's very easy with Lee's mobility and Dragon's Rage, simply kick your target into your team, dragon, or baron to let them finish them off! Play Video
Juke walls.
Lee's kit, especially Q and E, were made for juking, jumping walls, because who needs open paths? Play Video

If laning, take your Sonic Wave/ Resonating Strike first, it gives high early game damage, as well as a good gap closer if you're against a ranged champion.
Take your Tempest/ Cripple second, especially against a bruiser like Fiora or Master Yi who rely on auto attacks, and have no good escape early on, so you can slow their attack and movement speed to crunch even harder.
Double tap your Safeguard/ Iron Will for the lifesteal and shield in exchanges if needed.

Don't be afraid to use your skills to farm up all you need, as they have relatively low cooldowns, and you can farm much faster!

In Season 2, it was norm to take your Tempest/ Cripple first, as it does AoE damage to the jungle creeps, but the Season 3 changes to the jungle made Sonic Wave/ Resonating Strike a much better choice for first skill, providing more damage output, and more useful in fights if invaded.

Start red buff, smiteless if you can, then rush over to blue at level 2 to give the buff to your AP carry in mid lane, which will help them greatly in using their abilities.

As soon as you hit level 3, grab your Safeguard/ Iron Will, and you're ready to gank, and only gank, repeatedly, Lee Sin has very high damage early game, and can enter lanes easily to provide the support they need.

Try to give the kills to the carries, rather than take them yourself, as Lee Sin's damage falls off late game, so it would be better in the long run to feed your other lanes.

In terms of counters, few pose a threat to a properly played Lee Sin, I'll be mentioning 5 of the biggest counters here, and how to deal with them.

Trundle:
The main counter here is Trundle's attack damage reducing Chomp, which whether in lane or jungle, devastates Lee Sin early game, making any trades almost impossible to win for Lee, as every one of his damaging abilities scale with AD, which Trundle saps.
Lee Sin is also tanky all round, which Trundle's Subjugate enjoys, as it saps Lee's armor, magic resist, and health, and transfers it to himself.

WHAT YOU CAN DO:
- If an exchange must be made, do not do it near minions, as Trundle's passive King's Tribute heals him whenever a nearby unit dies, which - if you didn't know - happens quite often to minions every game.
- Try to avoid said exchanges.
- If building damage-y, head more for armor penetration such as Last Whisper and Black Cleaver rather than straight attack damage items, which Trundle will steal from you anyway.
- If laning, wait for jungler's help before engaging Trundle, to catch him off, as he has no escape besides his pillar and speed boost, and a jungler such as Maokai, Jarvan IV or Malphite can reject those and pin down Trundle easier.
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Udyr:
The counter here is that Udyr has no poke, good gap closer, or much escape. However, when another closes the gap to Udyr, it's game over. Lee Sin's only damage output can be made when closing the distance between himself and Udyr, a simple Q poke would easily get healed off by Udyr's Iron Mantle. Once Lee Sin closes the gap, Udyr will make him pay with a Blazing Stampede smack to the face, and a facemelting Wilding Claw hit or two. Lee Sin does NOT want to make this tradeoff.

WHAT YOU CAN DO:
- Build more tanky so you can freely farm easier in lane, or make it out better in the jungle without too much trouble from Udyr.
- While Lee Sin is very strong early game, Udyr's Wilding Claw is also very strong, this combined with his stun from Blazing Stampede makes him superior early game, an exchange is not recommended. Avoid any confrontations.
- As mentioned earlier, Udyr has no real good escape besides his speed boost. If laning, you should try to get a jungler such as Vi, Xin Zhao or Amumu who can close the gap and keep him there, AS WELL AS DOING ENOUGH DAMAGE to take him down and shut him down easier.
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Renekton:
This counter is more seen in top lane, as Renekton - albeit possible and viable - is rarely seen in the jungle. Renekton wins all early game trades. Hands down. His Cull the Meek skill heals any and every poke done to him, his Ruthless Predator stun makes you never want to go near him again, and just as you think you've dashed to safety, incoming Slice and Dice to kill you again. Just when you think you MAY have killed him. Bam goes Dominus ult giving him what seems to be a thousand health.

WHAT YOU CAN DO:
- Try levelling your W first, for shorter cooldowns, more shields, and more sustain.
- If in lane, and possible, get a jungler who has good crowd control and % of health damage to take out Renekton, Elise or Jayce would be good.
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Elise:
Elise is an all-round bully, in lane, jungle, ganking, anywhere. If you're stuck in lane against her, or caught in a jungle fight as Lee Sin, Elise'll happily chomp you for dinner. Once Lee does for the dash, she shoots her Cocoon at you, stunning you only to launch her entire bursty combo, then should Lee try to run away, she'll pop spider form an Rappel back to you.

WHAT YOU CAN DO:
- Max W to reduce damage from her easily seen Volatile Spiderling
- Build more magic resist, and a Hexdrinker, as health will only increase her damage from % of health attacks.
- If you're forced into a fight with her, E as soon as her spiderlings are seen, to damage them. A Ravenous Hydra is also very useful at taking them out, they do more damage than they seem.
- If laning, try to get a jungler on your team who has a silence, or good gap closer with heavy damage, try Wukong or Cho'Gath for good effect.
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Nocturne:
This counter is mostly seen in the jungle, where Nocturne is best at. A well played Nocturne will use his Shroud of Darkness to block any kicks to the face Lee Sin tries to do. As soon as Lee closes the gap, Nocturne will use Unspeakable Horror to fear then go in for the kill. Should Lee kick him away or escape, Nocturne will Paranoia to close the gap, finishing off the poor Blind Monk.

WHAT YOU CAN DO:
- Try to out-time Nocturne's spellshield to land a Q>R>Q combo, and build more damage to burst him down with that faster.
- Level E to slow Nocturne's attack speed and movement so it'll make it harder for the flying dustball to catch and kill you.
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I'm no master, but I hope I helped. Best of luck with your Lee Sin'ing and killing all your enemies with monk-ish style.

Force is meaningless without skill.