Cassiopeia is an AP Mid with amazing harass and damage. However, she is very hard to play and requires long term commitment.

PROS:

-Can easily control a lane and zone out opponents.
-Devastating damage and great at chasing.
-Outscales people.
-Really good Stun Ultimate!
-Absolutely amazing at harassing!

CONS:

-Really, really squishy with no escapes.
-Really hard to play well.
-The more you play, the more combo's and tricks you learn. Long commitment.

Deadly Cadence After casting a spell, any subsequent spellcasts will cost 10% less for 5 seconds. This ability stacks up to 5 times. Makes spamming less costly? Great!

Noxious Blast After 0.6 seconds, Cassiopeia blasts target 75-radius area, damaging enemies over 3 seconds. Cassiopeia is granted a movement speed boost for 3 seconds if she hits a champion with this spell.
TOTAL MAGIC DAMAGE: 75 / 115 / 155 / 195 / 235 (+ 80% AP)

Miasma Cassiopeia throws a coud of poison to a target location that poisons and grants vision. Understand that this spell slows and poisons immediately, letting you use Twin Fang faster than Noxious Blast.
TOTAL MAX MAGIC DAMAGE: 225 / 315 / 405 / 495 / 585 (+ 135% AP)

Twin Fang Cassiopeia's signature ability, Twin Fang. She throws her fang at an opponent, dealing damage. If the target is poisoned when hit then Twin Fang's cooldown is reduced to 0.5 seconds. This is the ability you spam for kills if opponents are poisoned.
MAGIC DAMAGE: 50 / 85 / 120 / 155 / 190 (+ 55% AP)

Petrifying Gaze Cassiopeia's hard to land ultimate...Cassiopeia deals magic damage to all enemies in a cone in front of her. Enemies facing her when damaged are stunned for 2 seconds, while enemies facing away are slowed by 60% for the same duration.
MAGIC DAMAGE: 200 / 325 / 450 (+ 60% AP)

Early game is very simply put as: understanding the nature of the lane -> how you're going to play. Are you facing a melee champion that you can easily continue harassing? Are you facing an enemy that outranges and have to be in a farm lane? This aspect is important because of your first buy. Are you going to be going Kage's for Twin Shadows? Do you need tankiness and sustain so you're going to buy Hextech Revolver?

Noxious Blast or Twin Fang

Twin Fang does more damage.

Lets say you have 200 AP, you land 2 Noxious Blasts and 4 Twin Fangs. You have 4 points in Q and 2 in E. Without factoring magic resist:
-2 Noxious Blasts = 2 * 195 = 390 + 2(200 AP * .80 AP Ratio) = 390 + 320 = 710 Damage.
-4 Twin Fangs = 4 * 85 = 340 + 4(200 AP * .55 AP Ratio) = 780 Damage.

Twin Fang does more damage. But I max Noxious Blast more. Why?

The strength of Cassiopeia is at levels 3-5. It's not at level 2. A level 1 Twin Fang does little damage. The chances of you picking up a kill at level 2 is really unlikely. Understand the nature of the lane and think about putting more points in Q or E.
---Is the opponent close range? Put more points in Twin Fang! You know you get to land more Noxious Blasts and get more opportunities to frequently damage with Twin Fang.
---Is the opponent longer range or playing passive? Not able to combo? Put points in Noxious! Putting 2-3 points in Q really early is underrated. When I'm zoning hard, all I have to do is land a silly poison cloud and I'm able to conserve mana. The opponent can't farm because he's scared of my damage, and I haven't even used Twin Fang.

Miasma does a lot more damage than people think.
MAGIC DAMAGE PER SECOND: 25 / 35 / 45 / 55 / 65 (+ 15% AP)
TOTAL MAX MAGIC DAMAGE: 225 / 315 / 405 / 495 / 585 (+ 135% AP)

To me, the difference between a good Cass player and a great Cass player, is how they use Miasma.

Use Miasma First!

Cassiopeia is known for her Noxious/Twin Fang combo, but Miasma just makes it even better. Throwing out a Miasma does a few things:
1. Lets you throw out Twin Fang faster: Miasma procs poison immediately unlike Noxious.
2. IT SLOWS. IT SLOWS: Did I mention that it slows? This is the reason Cassiopeia is good against melee champions. A slow from Miasma lets you hit Noxious Blast easier. It slows the opponent, and if you have Rylai's slows even more.
3. Lands you kills better: If you need to burst down, Miasma can only help. Extra damage + slow only helps.

Use Miasma before Ultimate

Using Miasma before stunning makes the opponent stay in Miasma's circle longer, dealing more damage. After the opponent comes out of the stun, they're slowed, letting you burst down easier.

Land Miasma behind Opponent

I like it when I move close in lane, the opponent starts to move back but I throw out a Miasma behind them as they turn around. They're slowed, poisoned, I get to land a Noxious easily after I Twin Fang. Easy harass and can pick up kills.

Boots + 3x Health Potions are standard opening items for Cassiopeia simply because it helps her harass easier and move around more.

Are you facing a melee champion? Use Amplifying Tome + 1 Pot. Harass him. Punish him for farming. Is he close? Use Noxious Blast I love this start because I have such early game dominance. I can just zone out melee farmers and control the lane completely just with Noxious Blast.

When it's going to be a farm lane, Faerie Charm + 6 Pots + Ward is my favorite start. Do I need to stay in lane nonstop and can't harass? This is the start to go.

Twin Shadows v. Rylai's Scepter

• Twin Shadows is a lot cheaper, gives magic resist, gives movement speed, and a great active. I love this item for early/early mid games or if I'm roaming a lot. It really helps pick up kills. Perfect active when the opponent flashes, I just pop the active to kill or get away.
• Rylai's Crystal Scepter : It's up to you to decide when you need health or a constant slow. Rylai's has always been a good item for Cassiopeia but I feel Twin Shadows gives enough slow that is needed for much of the early/mid game. Rylai's synergies well with Liandry's Torment.

Items to Think About:

Your items for each game with Cassiopeia is just going to be:
- for more Damage.
- for more damage and burning effect.
- or for the slowing effects.
- or : If you need tankiness and sustain, this is your item.
- or : If you need more MR or Armor, you think about either of these two.
- Sorcerer's Shoes.

I strongly recommend against and . They slow down your early/mid game dominance and only hinder you.

: Skill based matchup. It's about trying to get an early advantage, dodging her charm, and being aggressive at levels 3-5.

: Easy to lane against. If she wants to farm, punish her with Q-E combos. This is one of the matchups when I would recommend leveling W earlier. You can lay a W on top of yourself or on her shroud.

: Really easy since Anivia moves so slow and is so squishy. Easy to pick up kills on. If she wants to get close, punish her since Anivia's stun is easy to dodge.

: You don't see many Annie players but when you see one, the chances are they really play Annie a lot and well.
Annie is tough since she can stun and burst down so hard. It's tough knowing when you can be aggressive. I like to turn this into a farm lane depending on the skill of the player.

: Dodge. And keep dodging. It's about dodging his spells. If you dodge, you can zone him out. If you don't dodge the spells, the game will only get harder.

: Really hard to lane against since she's so tanky with her shield. I would just recommend farming the best you can since she pushes so well. Would really recommend Rylai's + Hextech + Abyssal.

: Easy. If she wants to farm, punish her. If she doesn't, just make sure you let your team know so she can't roam for kills.

If she stays in lane, you can decide between maxing Q or E. Q zones her out and doesn't let her farm at all. I recommend maxing E if she's dumb and wants to stay close. Punish her and combo for easy kills.

: If you do see a Fiddlesticks mid, just out DPS him. If he wants to drain, you can ignite, ult stun, kill. He's not going to be able to be aggressive without getting heavily hurt and risk dying.

: It's hard because it's about Fizz knowing when to use his pole/trickster. Don't use your ultimate if his E is up. I recommend at least 2 points in Q and would recommend Will of the Ancients + Giant's Belt. It's about surviving the initial burst.

: Gragas is about bursting. He's not going to use his ultimate without being in close range so he can body slam. You might want to consider early Negatron Cloak. Try to keep moving around to dodge barrels and land as many Q's as possible.
: Bruisers hit hard, really hard. Would recommend Twin Shadows to slow, maxing Q since you can't really afford to fight.

: Easy. He's slow, bad early game. Go full aggressive and just kill.

: It can either be a really easy lane or a hard lane depending on how Kassadin uses silence. If he farms, punish him since he's melee. Keep landing Q's and try to push. It's about not letting him roam at all.
Understand the range of his rift walk. He's a melee farmer and it depends on whether you want to max Q or E. If he's going a little safer, Q. If he's dumb and wants to fight early, max E.

Banshee's Veil helps a lot. If he's smart, he'll Rift Walk and then silence. If he's not and silences first, well just enjoy a kill if possible.

: Easy lane since Katarina is melee. I would level W early. Understand that her Shunpo moves her behind you. If she Shunpo's when you Q, pop your W and just punish her with Twin Fangs. If she ultimates, fire off Petrifying Gaze and kill!

: Understand when Kayle's Flaming Sword is running out. You can out DPS her early before Kayle buys a Nashor's Tooth. I like to use Noxious Blast a lot since you want to get her to try and use her heal spell to waste mana.

After level 6, you have to be careful since her ultimate gives her invulnerability. You should be able to out range and out harass with Noxious. Don't let her keep harassing with her Q + E, it can really hurt with just something like Nashor's Tooth and Large Rod or Haunting Guise.

: It can be easy or a little hard. It's about landing Q's and not letting Kennen stun. If he wants to move in with Lightning Rush, back out a little and just try to keep landing Q's. I would really recommend an early Twin Shadows or at least well on your way to building Rylai's Scepter.

: Keep moving. Always move around. Don't let her hit you with her Light Binding snare. Keep in mind (!) that her cooldowns are high early game and that her spells cost a lot of mana. If she fires off an E, dodge it and MOVE IN. If she fires off a Q -> E combo, DODGE IT and MOVE IN. Noxious Blast

After she uses her spells, she can't do anything but waste mana shielding. Punish her for having long cooldowns and high mana costs. Always keep moving.

: Well he's a lane bully and can farm safely with his E. I max Q here since he can keep his distance and farm easily. If he hits you with his silence, well you can't do anything but take damage from his E.

Most of the time, this is just a farm lane. You don't really want to go in aggressive since he can silence and also suppress. He's good at 1v1's for a reason, no point in trying to pick up kills.

: Not many play Morgana anymore but she can easily stop you from killing her. Many players can just put 2-3 points in spell shield and you're useless. Can't really do a lot except from making this a farm lane.

: Stay behind minions and keep harassing with Noxious Blast

: Easy to lane against if you just understand how she works. It's easy to harass her with Noxious, dodge her balls and just Noxious Blast and Twin Fang.

: Just keep harassing and farming. If he wants to go in, Petrifying Gaze, pick up easy kill. If he wants to stay and farm, keep harassing with Noxious Blast. Zone him out. He can't do anything but risk dying if he wants to farm.

: 2 items to get: Hextech Revolver/Will of the Ancients and Twin Shadows. If he wants to go in, ultimate + stun, combo, throw out active from Twin Shadows.
Keep in mind that you can't let him harass or snare you.

: Probably Cassiopeia's hardest lanes. You can't harass her because she harasses faster at a LONGER range. This is probably one of the times I would pick up either Tear or Chalice. It's unlikely you're going to get in range because she can stun fast. Syndra can burst really hard so Cassiopeia has to play it safe.
Really nothing to do but play passively and farm well.

: Twisted Fate can't do much to Cassiopeia. His Wild Cards is easy to dodge. If he wants to actually go in and lane a gold stun card, punish him. You can just land a Q and run out of range.

: Well, can't really build AP. You can harass at low levels well but his stun can easily pick up kills for his jungler.

: Easy since he can't afford any close range fights. Pre level 6, just keep harassing if he lets you. I don't recommend staying at long range from him or else he can just W - Q for long range harass.

If he uses his Q to farm, it's going to push the lane. Let him push since he's squishy. If he's auto attacking, just keep harassing.