Hi. This build is changing slightly since I had first posted it. While doing some calculations, getting 21 into the offensive mastery tree is ALMOST POINTLESS ON ANY CHAMPION. The extra 5% is only added on your base attack or ability power, and is not affected by any items. Basically, if you have 100 BASE attack damage(NOT INCLUDING RUNES, MASTERIES OR ITEMS!!), and 300 attack damage with items, you would have a total of 305 attack damage. The extra masteries were placed elsewhere to benefit people more. I would not recommend going 21 into offensive on any champion as it does not benefit anyone at all. Thought I would explain why I did this a bit.

ITEMS: Last Whisper/Brutalizer

ARMOR PEN ARMOR PEN ARMOR PEN!!!

With the recent update to BF swords having their cost reduced, it is much easier to purchase a black cleaver. I would only recommend getting a Phantom Dancer if you bought Merc Treads, or if you are needed to backdoor towers. Otherwise, ARMOR PEN WRECKS EVERYONE. Nuff said.

EDIT #2:
I messed up some of the sections (items and masteries) since i changed them in the sections above. Thank you for pointing that out.

This is Caitlyn. She might be your favorite sniper. She might be your ex girlfriend. Whatever she is, she is a long range physical DPS character with massive amounts of damage and utility. This guide is basically what I use for items, and what my playstyle is. Copy it and you have a dangerous DPS on your team that the other team should fear.

For runes, I typically use armor pen, mana regen, and attack speed. You may find flat health quints will help with early survivability, but normally you should be fine with armor pen quints as well.

These masteries are 19/0/11. Why? Because going 21 into the offensive section is pointless, it adds +5 damage to whatever your base damage is (so usually its around an extra 5 damage) which doesnt amount to much. Putting more points into support helps your mid to late game, with extra buff duration and some gold. Much more important than adding 5 damage to your auto attacks.

Initially, starting off with boots (and 3 health potions) is ideal. Doing so allows you to run up, harass, and run back before the enemy can react. Afterwards, buying 1-2 dorans blades will give you a longer lane presence, allowing you more health and some lifesteal on top of damage. However, starting with a doran's blade is great if you want to get that early kill, rather than harass until they die. next, finish your boots and grab a vamp scepter so you can lane EVEN LONGER!!! Rushing a last whisper is usually what I end up doing, because on top of the damage it gives, armor pen is always good. The less armor they have, the more damage you deal. In my opinion, it is just an amazing early/mid game item and will allow you to destroy even tanks. The BF swords should be built into whatever item your current game calls for. Bloodthirster if you can easily farm creeps and build up the damage counter on it. Black cleaver if armor is an issue. All other situations should include the Infinity Edge. Any of these options are good, but remember. They are situational. Buying more than one is fine too, such as infinity edge and black cleaver, or black cleaver and bloodthirster. just remember, with all this damage, the other team will probably catch on and focus you. make sure you position yourself behind your tanks during teamfights, while bashing at their source of damage. Finally, a defensive item will be the bane of the enemy carry's existence. If you put 2 caitlyns in a fight against eachother, one with nothing but damage, the other with half that damage, but a sunfire cape, the caitlyn with armor will always win. not only does it help you survive, but it lets you live long enough to fight off any threat (unless you get crowd controlled)

Passive- Headshot:
This is perhaps one of the greatest harass passives in the game. After every 7 shots, your next shot deals bonus damage (150% to a champion, 250% to a minion). This passive is doubled if you are firing from within the brush (ex. 2 shots in the brush count as 4 shots outside of it). A tip for getting the passive charged quickly is 3 shots in the brush and one outside of it. Then make your way to the nearest champion and fire. If they back off, attempt to chase them outside the range of experience. If you fire on a champion, back up and recharge your passive again. However, headshot is also an amazing farming tool. Early game you can kill minions that are at half health if you have headshot charged. Just remember to use it wisely. Using it to farm gold is great, but if you do 100-200 damage with a single shot at level 3 or so, you have an amazing harass there.

Starting out with Piltover Peacemaker lets me have a great harass as well as easy minion farming starting at level 1. Careful not to spam it however. I see far too many Caitlyns spam the peacemaker and end up with no mana during fights to be useful. Level this one up whenever you can.

Getting the Yordle Snap Trap at level 2 allows me to be ready for any gank. The best places for those are in bushes that enemies normally hide in to gank, such as the one near dragon, near baron, the 2 bushes next to mid, and the bushes just behind your tower. Since they are easy to see, throwing them in the middle of the lane is [usually] useless, because they can just walk on them and walk away, and the trap is gone without anything good coming out of it. I say usually because if I am laning with someone with a taunt or heavy slows (nunu, shen, rammus, etc) then setting traps where they will go to escape is a great idea. Only put one level into that until late game, since the damage is minimal and the snare lasts the same amount for each level.

Next is the 90 caliber net. This is one of the best escapes in the game, as it lets Caitlyn have a free flash that also slows any enemies it hits. It can also be used to help chase down enemies, allowing you to throw your net behind yourself to jump a bit further ahead. You may also jump over walls with this skill.

Last but not least, Ace in the Hole. This is a lock-on shot, never misses, and always deals massive damage to a single target at a huge range. However, allies are also able to jump in front of the shot to block it. The best times to use this are when there is a single low target without allies nearby, or a few targets that will all die if they attempt to intercept it. The other good time is when the enemy team is running from a team fight. If any targets are low, lock on to them. Normally, they all are too distracted with running away to help their low ally, granting a free kill.

I don't believe there are any bad summoner spells... I just believe some are better than others. Here is a list of what I feel to be the most useful to the least useful on Caitlyn.

1: Ghost- Almost a must-have for Caitlyn. Being able to run to/from targets is amazing, letting you chase for that easy kill, or escape potential death.

2: Flash- Great utility skill. Jumping to the perfect position to or from targets is great, and you can never have too much survivability.

Those are the 2 spells I take as Caitlyn. Here are the others...

3: Exhaust- Great for 1v1 fights. they deal less damage and are slowed for the entire duration.

4: Ignite- Good damage skill. Allows you to finish off targets (if your ultimate is down or you don't have enough damage with it alone). However, you are also a sniper. Being close enough to use this skill is dangerous in team fights especially. Use it wisely.

5: Teleport- Lets you get around the battlefield. Good for holding your lane (yay laning phase)

6: Cleanse- Clears yourself of debuffs. Nice escape mechanism.

7: Heal- Useful if you are low on health, or want to trick your opponent into thinking they can kill you.

8: Clarity- Want to spam your Peacemaker? Clarity is nice. Not much use once midgame begins however.

9: Clairvoyance- Good scouting tool. Better on a support however

10: Fortify- Enemy caster minions giving your towers a problem? Fortify it! Pretty useless on Caitlyn.

11: Smite- Not the greatest spell for Caitlyn, but helps clear minions and buffs faster. Caitlyn cannot jungle however.

12: Rally- More damage. Yay. Useless after the begining of the match.

13: Revive- You shouldn't die. Revive is a babysitter for people who die too much. STOP DYING AND YOU WONT NEED REVIVE. I hate revive.

Lets talk about how Caitlyn plays. I explained her skills and how they work. Lets put them to action. Try to grab mid if you can, unless there is another carry on your team who needs mid. If you can't mid, TRY YOUR BEST TO LANE WITH A TANK. You can do enough damage to piss anyone off on the other team, you just need a partner to soak it up for you too.

Early game:
Harass enemy champions with your peacemaker IF YOU HAVE A GOOD SHOT. Don't waste it by trying to aim for them and hitting nothing. If you dont have a clear shot, hit the minions with it. Put snap traps in the bushes where ganks will come from and save your net for an escape. Get as many kills as you can, and ask your laning partner to back off if you are going to score a kill with your ultimate.

Mid game:
Team fights are starting. You should have your phantom dancer by now, maybe your black cleaver. focus the enemy squishies and throw your peacemaker out for AoE damage. Snipe anyone low with your ult.

Late game:
Items should be complete/close to it. play similar to mid game, focusing squishies. Stay in the back of your team though. You dont want to be caught in the middle of the fray.

End game:
Victory. Enough said.

When I look at this build I notice the good parts and the bad parts.

Pros:
AMAZING DAMAGE OUTPUT.
Long range harass and ultimate.
Fairly easy to play/learn.
Good utility.

Cons:
Not as much support as other ranged DPS (ashe's stun + slow, Teemo's mushrooms)
You should get focused a lot.
Not much defense, only escapes.

This is my build, and how I play Caitlyn. Comments and questions can be submitted below. I play Caitlyn a couple times every other day or so, so if you want, check my match history on how I personally do with her. Other than that, Good luck, Kick ***, and Snipe Noobs.
-Peakreaper