Caitlyn is in my opinion one of the most powerful AD carry of the game, with a safe early game in which you've to farm as more minions as possible you will slain everyone in an 1vs1 or in a fight match. So many people will say "naaah, caitlyn is so squishy, she can lose with everyone". Exactly, she "can", but I don't lie if I say that with her spells she can manage every enemy.
W and E spells will help you more than you can expect, especially to escape from "bad situations".

How about runes? For marks I used Greater Mark Of Desolation who grants me a nice damage vs every Champion. Combined with Greater Quintessence Of Desolation I reach about 25 Armor Penetration.

Seal's selection can be so much variable, depending on how you can manage her during the match. As a squishy champion, some of you should choose a Greater Seal of Armor, but I prefer to give her some pure atk damage with Greater Seal Of Strenght. Some months ago I had Greater Seal of Attack Speed, but I noticed that with these additional points on Atk Speed I've a more confortable early game.

And Glyph? Don't feel offended if I don't put Mana regen runes. With some train you'll be able to play without them. So my choose is Greater Glyph of Scaling Magic Resist , 'cause I want to have at least some little defense vs mages, considering that I'll be the main target in team fight

Not so many things to say about masteries. That's the best choice for an AD carry, I need a powered Exhaust and a fast leveling. 21/9/0 grants you a quite safe early game and a powerful mid/late game

I prefer to start with a Doran's Blade, cause I need to farm and resist in lane as more as possible, and this is the most confortable item for it.
During our first recall (depending on how many golds I have) we will buy Berserker's Greaves and Vampiric Scepter. With life steal I want to grant that I'll go again in base just when I really need to buy items and not 'cause I'm death or I've lack of HP.

Now that we have a good mobility and some useful items it's time to build our firepower. I saw in some guides that people instantly buy Bloodthirster , but I prefer to take advantage from Infinity Edge passive. Believe me, in this way you will hurt enemies both with skills and autohits.

Now that your damage is consistent, how about some critical+speed? So, it's time to buy a Phantom Dancer that allows you to have additional movement speed too (in this way you'll avoid a lot of ganks).

Ok ok, now it's time for Bloodthirster , and with Phantom Dancer you'll farm minions faster and you'll have your bonus stacks in safe, 'cause it will be hard to kill you.

Don't hesitate and buy another Phantom Dancer to be a real war machine.
And if you don't feel in safe? Well buy a Banshee's Veil .

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Notes: Versus Tanker Replace Phantom Dancer and Banshee's Veil with Last Whisper and Madred's Bloodrazor.

In summary:

EARLY GAME:




MID GAME:



LATE GAME vs BALANCED:



LATE GAME vs TANKER OVERPOWERED

As a Carry AD I don't have to let my enemy to think or run, so my choice will go into and .

In this way I can use them both as attack or defensive spell to make me follow a champ or to make me run from him.

Skill sequence seems pretty obvious.
is our main skill, so let's grow it as fast as possible!
But don't undervalue (especially in 3vs3) and . You need them to kill and run easily from your enemies.

If you wanna play Caitlyn don't be afraid to face every Champion. Remember that you've one of the most powerful AD Champ of the game. And I bet that with this guide there will be only a scared guy: your opponent.

Edit: 26/06/12 refreshed the build + runes
Edit: 19/02/12 refreshed masteries and runes