Hello evryone,I play a long time Kennen(it's my main)and i show you my guide.This is my first guide so try in game before voting :)

Pros:
Aoe Stun
Good Harraser
Easy Farming
Good Dmg
"Ghost" in skill
Easy to learn how play him
Cons:
Squishy
Most focusing
If you have skills on cd you can be easy killed

My masteries is 21/9/0 but sometimes i play 9/21/0.It's a lot way for Kennen and defense tree gives you more survivability.
This is secondary mastery tree:
http://www.mobafire.com/league-of-legends/mastery-tree-planner#&tree1=0-0-4-0-0-4-0-0-0-1-0-0-0-0-0-0-0&tree2=0-3-1-0-4-3-2-1-0-0-0-3-0-3-0-1&tree3=0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0&v=2

Greater Mark of Insight
Great choice if you wanna play ap kennen
Greater Seal of VitalityGreater Seal of Vitality
More hp for kennen ? why not,this runes gives you more survivability
Greater Glyph of Focus
You can also take Glyph of Potency but i prefer have more cd and faster harras enemies
Greater Quintessence of PotencyWhat can i say,ability power gives you extra dmg in early game

You need to max skill because you can harras enemy with this skill.Then and last

Good Spells:
Flash Great spell for any ap caster so for kennen too.This spell help you espace or catch up enemy.
Ignite Awsome spell too,extra dmg helps you kills running enemies.
Exhaust Good spell,earlier i use this spell
Cleanse Good spell if you really hate cc :)
Medium Spells:
Surge Well bonus ability power and attack speed not bad,but you dont hybrid and you dont need attack speed
Ghost If you really need this spell take it but kennen have ghost in skill
Teleport Trust me you dont need teleport
Bad Spells (no comment)
Smite
Heal
Clairvoyance
Promote
Revive
Clarity

Farming as Kennen is very easy.Start at skill on minions and then Play Video

Try harras enemy before team fight with ,remember your role in team is dealing huge dmg and stun enemies.Start at skil and then With your passiv try stun all enemies around the strom with
Here is some videos explain you what a mean: Play Video Play Video

This is core items:




If you have problems with:
Ad carries and Fiddlesticks ulti etc.:

Ap takny/nuker:


Others:
If you pwn enemies and you are too strong to die you can easly take Mejai
Not bad cd reduction is always helpful but i prefer

Kennen is a great champion who can do a lot combos with ultimate like Nunu or Amumu.Remember to harras enemies and stay on your team.Thanks for reading,good luck and see you later!

01/02/12 I added secondary mastery tree and correct "pwn the enemies" :)