Hey summoners, Cmarty here. This is a full ap mid guide to Katarina. In this guide I will go through item choices against different match ups, when to build different items, masteries, runes, and why I prefer not building certain items on kat! (Like Gunblade). I hope all that read this guide will learn valuable things. Please feel free to comment and don't be afraid to voice your own opinion! :)<3

Abbreviated Terms
CC = Crowd Control (Stun, fear, slow, snare, etc)
AP = Ability Power
AD = Attack Damage
MPen = Magic Penetration
CDR = Cooldown Reduction
OOM = Out of Mana
HP = Health/Health Points/Hit Points
DoT = Damage Over Time
AoE = Area of Effect
MRes = Magic Resistance
CS = Creep Score
MS = Movement Speed
DPS = Damage Per Second

Beautiful Pros Ugly Cons
:) 0 mana problems
:) Amazing passive in VORACITY
:) Mostly Aoe damage
:) Not hard to learn, but somewhat difficult to master
:) No Skillshots
:) Shunpo allows her to be extremely mobile
:) Good wave claring mid game with Bouncing Blades + Sinister Steel
:) Strong ganker/clean up.
:) Death Lotus can show if invisible targets are in range.
:) Healing Reduction ultimate.

Ugly Cons
:( She gets focused and CCed A LOT, you really need to know how to position yourself and when to enter a fight.
:( Melee range in mid. Auto attack harrass OP.
:( High early game cool downs.
:( Need to pick up kills/assists in team fights.
:( Weak early game.
:( Terrible base and scaling

While playing Katarina in the mid lane, there are different item builds you will want to consider.
Starting, I personally always like to start 9 health pots and 2 wards. Having the 9 pots you want to keep trading your opponent and force them to back before you do. With the 2 wards you will almost have certain safety from a gank. Say you ward the other teams wraiths and the jungler still comes in from the other side you still have a extra ward to Shunpo too incase you get caught.
Against physical damage dealers though I usually start cloth and 5 health pots to rush the Seeker's Armguard
Boots and 3 Pots, The only time I will ever start this is against a lane that is easy to deal with and having the extra mobility early with benefit me. Ie Karthus

Guise - When you have a even lane and want to get a early advantage pick up a guise. The extra Magic pen is gives you along with the extra ap and hp it will help you out tremendously but in late game sell it.
Boots - Usually always rush Magic Pen boots on Kat, unless against a AD heavy team then go ninja tabi, same thing goes the other way, if facing a heavy CC team or say a veigar go merc treads.
Blasting Wand - What to do with that blasting wand? There is a couple way you can go with it! If you are in a hard lane against someone who can really harass you hard and out damage you go abyssal with that blasting wand. Having that MR and the extra magic pen from the aura it will greatly help you out. If you are doing well, having a easy time, rush the void staff. With the way Riot has changed how magic pen works just a void staff will give you more damage than just a deathcap.
So After you decide what you are going early on, either it be boots, guise, abyssal, or boots into void, I would suggest going deathcap. You badly need damage on Kat cause you can't rely on her base. After you get your deathcap, assess what items you need next. If you want more survivability and damage go for a hourglass. If your opponents have tons of hp then build a DFG. Against heavy CC teams for just pure defensive item I would build a GA and against not so heavy CC teams i'd build a Warmogs, but it personally your preference.

The way Katarina's kit is, some items just aren't viable on her. In this section I will go over what items and why I feel they aren't efficient.
Rylai's - Rylai just isn't strong on mid Kat. If you play top lane kat it is a mint item that is extremely strong on her, but playing her as the ap mid I want to be dealing as much damage as possible. First off, since Kat's skill set is mostly AoE you don't get the the full slow off Rylai's. Also in place on it I use Hourglass. Ask yourself would you rather have 500 hp in a fight or 2.5 seconds of stais? I think its pretty obvious.
Gunblade - I see alot of Kat's build Gunblade or Wota and its not as effective as it use to be. Like with the Rylai's she doesn't get the full effectiveness of spellvamp. Gunblade also gives AD which 2 of 4 Kat's abilities don't scale off of making it a very weak item.
Liandry's - Once again on top Kat this is a strong item, but this gives limited finishing power on her and since I am building her glass cannon I need to kill my target as fast as possible so I can get my passive off.

1-6 - Early on Kat is really weak. You want to focus on farming and harassing down your lane opponent everytime your Bouncing Blade is off of cool down. Don't waste your Shunpo to get in range to Sinister Steel unless you are 100% sure you know where there jungler is and using Shunpo won't get you into any trouble.
6+ - After 6 you get alot more powerful. Don't hesitate to use Death Lotus even if you won't get a kill because pressuring your opponent out of lane is almost as good! And by the time they get back to lane Death Lotus will already be back off of CD. Even at 6+ though try your best to keep farming. Getting ahead with Kat is really crucial, in levels and items. Try to avoid roaming unless you are 110% sure you will be able to do something. Don't leave lane trying to make the plays and fights because most of the time you won't be able too so you just end up missing out on CS and exp. ONLY ROAM WHEN YOU CAN GUARANTEE A KILL!

This is what seperates the Amazing Kat players, from the good Kat players, to the bad ones. Teamfighting with Kat in my minds is one of the hardest things to do correctly. Good positioning is 100 percent needed! You need to know how much damage you will be able to deal to the target you want to kill and the right time to go in, when to use your items, how to dodge spells with Shunpo. As Kat, if you go in to early the enemy team will destroy you, but if you enter the battle to late your entire team will be dead. When teamfighting try to go in after most CC have been used or you can get onto a squishy and get your passive off without getting interupted. You have to have really good judgement in this part of the game. It will devolop over time so if you are new to Kat and find you can't quite grasp this skill yet don't panic, you will eventually get the hang of it over time! :)
P.S. A quick tip for laning and team fighting! It's always nice to buy a red elixir for extra hp and ad and it gives you the ability to bait out the health increase.

I hope everyone who reads this simple Katarina guide can learn something valuable from it. I hope It will give you a new view on Katarina and hopefully you can destroy others as much as I do with her!<3 Please feel free to contact me if you have any questions about anything! And please leave a comment and let me know your opinion on this guide, good or bad! Thanks and have fun throwing knives! <3 :)