This is my first guide, so it'll get messy.However, if you want to understand Katarina as a champion it's gonna take more then a itemset and runes.This guide will describe when Katarina's strong, weak, when should you jump in a teamfight, when should you back up etc.I will do my best to describe what is neccesary.

I will edit the guide every day uploading new stuff and hopefuly you guys will like it.

Well you are probably wondering why did I go with these runes.9x magic pen is a classic thing, you'll find it everywhere, now 9x armor is good against laners like Zed, Yasuo, Talon, Kha'Zix and all those other assassins you might be facing.9x magic resist is good against all those pain in the *** champions like Ahri, Nidalee, Annie and whoever using AP you are facing.Of course it can be replaced with AP runes and seals with health per level.I've tried all these combinations out and resistance runes work the best on me, you can stay in lane longer since you take less damage (just a bit, but still less), you can survive longer in teamfights (that is what this build really is about, dishing out tons of damage while you survive) and thanks to these runes you can put more points in offense tree in masteries.

After being criticized a lot I've decided to take a deeper look at them and just take Expose Weakness but not Spell Weaving because I don't really find myself going in with basic attacks.And since Expose Weakness increases all allied champions damage to that enemy it's really beneficial with Katarina's ulti.I think they're pretty understandable the way they are.

I start of with Boots, 4pots and a Stealth Ward - that is a great thing,it's a free ward I use not to ward my lane, but to jump on it to escape or much more cool stuff !YOU STILL HAVE TO BUY WARDS!You can replace Boots with Cloth Armor if you're laning against someone with a lot of AD or you can go with that 5pots 4wards way, or a Doran's Shield way, which is a better start if you are facing AD champions but if you are facing a champion that has to land skillshots boots are better since they help you dodge.First back should always be Haunting Guise since it has HP, penetration and AP.That's what you are looking for in average.Core - of course Rabadon's Deathcap for the high AP Abyssal Mask for AP, magic resistance and that beautiful passive, Zhonya's Hourglass are always good on Katarina, you get high AP, armor and again, beautiful passive that saved my life many times.If enemy has no AP or their AP carry sucks then Abyssal Mask replace with one of situational items, even thought Deathfire Grasp is a good item, I don't really get it since it doesn't fit into this build, but if you're doing really well and have no one to worry about it's always nice to have that bonus 120 AP with the passive. Rylai's Crystal Scepter is also really nice on Katarina since you slow your opponents while using Death Lotus so they will have harder time running away. Void Staff has amazing passive so if I don't need Abyssal Mask I get Void Staff instead.Boots are really situational and depend on what situation you are in, so if you roam a lot Mobility Boots, if you can get your kills in lane and don't have to roam so much just get Sorcerer's Shoes and if enemy AP carry/AD carry is doing really good and you feel like they still hurt you a lot get Ninja Tabi or Mercury's Treads.Well now for the situational - If you're getting Deathfire Grasp as one of the core then you should look for more the assassin like build out here.I've mentioned it many times and this build focuses more on staying alive while you kill your enemies, don't get me wrong Deathfire Grasp is a good item, but make sure you get it only if you don't need the Abyssal Mask. Mejai's Soulstealer is beneficial on Katarina if you are doing really really good.You probably won't find it in ranked games really often but it's a great item if you have mastered Katarina and you are 100% sure that you won't make any fatal mistakes, if you are sure they can't really shut you, that you won't go crazy.It's a good item to prevent your deaths since, you know, you care a lot more, but you won't get it fully stacked if you just use it to pay attention or whatever. Will of the Ancients it used to be really really good but I don't know now.The fact that they added mana regen bothers me a lot, it's still good thought. Hextech Gunblade good too, has nice active, spellvamp AP and AD, fits more with Akali and Jax in my opinion but you can grab it.Not as good as the rest of items thought. Guardian's Angel I don't usually get it but it can be great in late game.Buying GA usually sort of requiers replacing one of your items with Deathfire Grasp because it helps you stay high with your AP.

Alright, you might be wondering why I max Sinister Steel before Bouncing Blades, well this is pretty situational aswell.It's not wrong to max 1st Bouncing Blades before Sinister Steel but I like to max Sinister Steel since it has lower cooldown and it's a great farming/harrasing spell. Bouncing Blades however are too. Sinister Steel does more damage and has lower cooldown, this is why I tend to max it first.It also has bonus movement speed when you hit champions with it and that helps you escape ganks. Bouncing Blades and Sinister Steel are both great choices to max first so I am ok with these as long as you don't max Shunpo first.And of course max Death Lotus whenever you can.

I choose Flash because you can Flash to get closer to your opponent, Flash to get away, Flash over walls to juke your enemies.
I choose Ignite to secure kills.
Other good options :
Barrier you can survive longer
Teleport you can do a Teleport gank, you can join teamfights faster
Never pick those :
Clarity you have no mana
Smite you are not the jungler!and if you are, you aren't playing Katarina!
Revive you are just asking to be killed
Not bad, but there are better :
Ghost isn't as good as Flash since they can slow you while you use it.You can't Ghost over walls
Clairvoyance you are not Nidalee, I seriously can't see how this can be used offensively if you're not ranged, maybe to see if they're doing baron?better would be just getting Scrying Orb
Exhaust so they don't run away from your ulti, never really tried it, can be useful
Cleanse kind of like barrier, it's nice to have this against high cc teams
Heal you can survive longer, but again, the cooldown is too long and it gives you just a bit of HP

Pros :
Awesome in teamfights
Great poke
Amazing farming ability
No cost
Has escaping tool
Seriously, that Slay Belle skin looks awesome
Cons :
Will get focused a lot
CC ****s you up
If you are 1st pick and you pick Katarina they will counter you 100% because they don't want you to be in the game, they are scared that you will get fed.

Ahri - she will piss you of with that Charm of hers and probably interrupt your ult if you don't play smart.She can heal herself well, she has true damage, she can run away from your ult with hers.Harras her with Orb of Deception, don't get hit by Charm, ult her only if her ult and Charm are on cooldown.This'll be hard for you.Just dodge and wait for gank, or roam.

Akali - you can ult her even if she tries to hide.You can harras her better pre 6.It'll probably be pretty even at times but you shouldn't have much problems if you play smart.

Anivia - her spells are really slow you should have no problem dodging them and you can jump over her Crystallize.Her Rebirth will be a pain thought.Don't underestimate her![/h1]

Annie - this little girl is pretty powerful you will have trouble with her stun.Harras her with Q, wait till she wasted her stun.This lane is about opportunities to kill her.

Brand - they say he's OP I say dodge his skillshots and you are perfectly fine.

Cassiopeia - same as Brand.Dodge her poision and ultimate if possible.

Cho'Gath - nobody really plays him mid but he can be a pain, he's tanky, he has knock up and true damage, he recovers when he kills a minion.You will definitely need a gank.

Diana - hey Diana, how about you pick someone your own size!I hate her on mid I seriously do and I hate being against her she has that Pale Cascade, good harras and when she gets blue nothing can stop her.However, if she doesn't feel free to harras her because she'll run out of mana soon.Watch out for her Moonfall before you start ulting her.Don't get hit by her Crescent Strike.

Elise - don't get caught in her Cocoon.She will be annoying and when you ult she can just Rappel and avoid taking the damage.You can harras her but if she doesnt make any serious mistake it'll be hard to kill her.Call for a gank maybe?

Evelynn - nobody plays her mid now but I am going to write this anyway.She has no way to interrupt you, however she gets a shield from ult and can run really fast so you probably need someone to stun her for you.If she engages on you then Q W R and if she runs E ignite W (just spam spells!)

Ezreal - He's squishy.Poke him and wait till his Arcane Shift is on cooldown, then kill him.Dodge his skillshots.

Fiddlesticks - He will Terrify and Bountiful Harvest you.He has silence.He will be a pain, but he's squishy, poke him, wait for gank or just wait when his fear and silence are on cooldown, or surprise him, or wait when he makes a mistake.

Fizz - I hate this champion!He can just jump away with playful/trickster, but you can dodge his ult.In this lane I would recommend roaming.You'll grab some kills and he'll die in teamfights eventually.

Galio - one annoying counter.You probably won't get any kills against him so again, roam, you probably won't do much damage to him and he can interrupt your ult easily with his.He's annoying, roam or wait for gank.Or if that Galio is stupid you can kill him.

AP Gankplank - he heals for a lot but it's AP GP.You should wreck him.

Gragas - his passive is annoying but again, it's about dodging skillshots.But beware of his ult.

Heimerdinger - he's squishy, towers can be annoying but you can dodge his rockets with your E pretty easily.Ult him when his stun is on cooldown.

AP Janna - Eye of the Storm and Monsoon will annoy you.She can interrupt your ult but she's squishy.If someone stuns her for you.If the lane is absurdly even and no one can kill another then just roam or call for gank.

Jayce - he can get pretty annoying, he will knock you back if you get too close and

can harras you.However, if his knock is on cooldown you can jump on him and start ulting just unleash your combo, test how much you do, if it's use to fight him, just farm and roam or call for a gank.

Karma - she has no option but to run.She can't interrupt your ult and if u dodge her Inner Flame that she will 90% of the time max done deal.Just wait 6, kill her, profit.

Kayle - hard one.Really really hard.Try killing her pre 6 with your jungler or just roam.A really good Kayle can wreck you in pieces.Her kite is so annoying, she can't interrupt you but she can become invunerable.Her Reckoning and Righteous Fury will be a pain.

Karthus - he's easy to kill.However beware his Death Defied so when you are diving make sure you have enough health to survive his passive.He's easy, but can annoy you with his Lay Waste.Boots are perfect start for this matchup.

Kassadin - You can kill him when he wastes Null Sphere.Can be pretty annoying and good Kassadin player will make you have some trouble but he has no interrupt besides his Q.He can also run away with his Riftwalk so make sure you ignite him and poke low enough before you ult so even if he tries to escape ignite will get him.Deny his farm you have better poke and you have no cost whatsoever while he's manahungry if he doesn't attack minions.

Kennen - dodge his q, try catching him while his E is on cooldown or he'll run away from your ulti.It takes some time till he stuns you in his so you should be able to put some dmg off.Zhonyas are really beneficial in this lane since you can deny all his ult dmg with them.

Kha'Zix - he's either really strong or really weak.He can escape you with Leap but that's pretty much all he can do.Since you have armor runes and bonus armor in masteries you won't have so many problems early.I've never really met a Kha that would win a lane against me.

LeBlanc - pain in the *** early.But if you don't feed her a kill and keep track of her if she's roaming to get her nothing will transform this pain into an easy kill.Try poking her while you are not silenced but dont engage on her.LeBlanc players can run away from your ult with Distortion, then trigger the Distortion again to come back, Mimic for silence Etheral chains, ignite and you are pretty much dead.She is dangerous but kind of like Fiora.If she doesn't get anything early she's weak.Sit tight for you opportunities or roam.

AP Leona - hard matchup.But you probably won't see it in ranked play.Just Roam she has two stuns and is tanky as ****.But again, I won't describe much of it because I went against it once.

Lissandra - she's tough she will build all this HP and mrez against you, she has powerful cc BUT has high cooldowns.Once her Frozen Tomb is on cooldown and she can't escape with her Glacial Path you have a chance.Ask your jungler or roam.She's difficult to kill but still can be killed.

Lux - if you dodge her skillshots then you are perfectly fine she has no way to interrupt you and if she doesn't land Light Binding and Lucent Singularity she can't kill you with ult.She's a late game mage so stomping her from the beginning shouldn't be hard for you (she has good range thought) but can't hurt you much if you don't let her.

Malzahar - this guy.He can interrupt your ult, he has that Call of the Void, he has this annoying little void pet running around and that annoying Malefic Visions.Dodge his Q, stay away from his W and distance yourself from enemies with E.You can't really kill him if his ult is not on cooldown.Roam or ask for gank.

Mordekaiser - will get tanky with time.Harras him early he can't really harras you back, only with Siphon of Destruction.If you don't give him opportunity to hit you with Mace of Spades (and since you have speed on Sinister Steel if you hit him he shouldn't even be able to).You should be able to kill him early.

---- MORE COMMING SOON ----

Pick when :
Enemy team has low CC
You have nice ult combo with whoever there
When not to :
Enemy team high CC
1st/2nd pick (let someone pick her for you when you see their team)
You got hard countered/you don't know how to lane against a champion