Recently there has been a resurgence in the popularity of Kog'Maw. I attribute this to his recent tournament play. Honestly he was one of my favorite champs when I started playing. I use to play Kog as an AP champ focusing on boosting his abilities but since his buffs, I find AS/AD to be a safer play style.

The reason why I choose Kog'Maw as my range AD of choice is because he is very versatile. His W gives you range and allows you to focus any target (due to the percentage damage). His Q reduces the targets resistances allowing for your team to benefit. His E places a "skillshot" AoE slow. Lastly by the level 16, his R has ridiculous range and gives visibility to any targets hit making juking into the bushes pointless. If you happen to die, Kog's passive can secure a kill or help turn the tide of a team fight.

All in all, he adds much to the team.

Pros

Deals crazy amounts of damage quickly
Great split pusher/farmer
One of the few champs that can focus tanks (still not recommended if the enemy carry is still up)
Extremely versatile carry
Spammable Ultimate that reveals enemies hit
Ridiculously fun to play
Awesome skins

Cons

Squishy (but really all carries are)
Susceptible to Crowd Control (again, most carries are)
Often focused early in teamfights
Always hungry

While I mentioned Kog's abilities as reasons to choose him, it's important to know what those abilities are.

Caustic Spittle Kog's "Q" - Passively gives (10/15/20/25/30%) bonus attack speed. Active deals(60/110/160/210/260+0.7 per AP) Magic damage and reduces Armor and Magic Resist by (5/10/15/20/25) for 4 seconds. I usually pick it up first for the passive and to harass enemy champs with the active (since the range is greater than your normal attack range).

Bio-Arcane Barrage Kog's "W" - Activate to increase Kog'Maw's attack range by (130/150/170/190/210) and all auto-attacks deal additional magic damage equal to (2/3/4/5/6% plus an additional 1% per 100 AP). This ability is why I use to play Kog as an AP champ. I max this ability first for the additional range but also for the extra damage (which can add up quickly if MR isn't built).

Void Ooze Kog's "E" - Spits out aimed "skillshot" ooze that deal (60/110/160/210/260+0.7 per AP) magic damage to all enemies hit and leaves a trail of void ooze that slows all enemies by (28/36/44/42/60%) for as long as they are on it (note: when used to escape it is best to aim in the direction you are running since pursuing enemies will be slowed while chasing you). I pick and max this last because for this build I feel his first two abilities are more important. Feel free to level this earlier for the additional slow.

Living Artillery Kog's Ultimate aka "R" - One of my favorite abilities. Kog spits in the air and after a 0.6 second delay the projectile lands dealing (80/120/160+0.3 per AP+0.5 per AD) Magic damage to all units hit revealing them for 4 seconds (base damage is tripled against enemy champs). Each use within 6 seconds (each use also restarts this effect) increases the mana cost by 40 (capping at 200 mana per cast). Why I love this ability is you can use it to scout the brush, reveal escaping enemies through fog of war/brush, scout neutral monster camps in enemy jungle, and poke turret huggers and so much more.

Icathian Surprise is Kog's passive. When he dies, Kog'Maw gains additional movement speed for 4 seconds before going 'splode dealing 100+25*lvl true damage. While you don't want to die, this allows you to still be effective in team fights after you die or to clean up a kill when an enemy when they're low. Sometimes this bugs and you just stay in place for the 4 seconds.

With the additions of new runes, and more importantly Quintessences, I've switched out the Greater Quintessence of Health for the new Greater Quintessence of Life Steal which gives 2% Lifesteal. However Mobafire hasn't included these new Quints so I have it bookmarked as Greater Quintessence of Spell Vamp (which is not the intended quint).

Because we can now start the match with 9% lifesteal (6% from the Quints, 3% from Masteries), I've switched from Greater Mark of Desolation to Greater Mark of Attack Speed for greater sustain. This also allows us to keep Mercury Treads and still get our end game attack speed past 2.0 attacks per second.

I've had something of a change of heart in seals as well. As I've stated in the past, the 2 favored seals are Greater Seal of Armor (which is now included as the staple in this guide) and Greater Seal of Vitality (which I previously favored). Both are good but the Armor early game can save you from early ganks and still scales into late game since Armor increases your effective health by a percentage equal to it's rating (with no additional health these runes effectively raise your health by 247.71 against physical attacks as opposed to a "flat" 174.96). The armor is certainly favored early game but it really is a personal choice.

Some people prefer Greater Glyph of Scaling Ability Power (0.17 AP per level) to the Greater Glyph of Magic Resist (1.49 MR) since it adds damage to Kog's abilities but trust that you'll want the survivability. Don't even go for the MR per level glyphs because it's weaker until level 10. Some people swear it's better for late game but I feel that by late game, you don't really need to worry about the minimal difference due to the crazy amount of Lifesteal you'll have.

Masteries are chosen for what I'd hope are obvious reasons. I don't go full crit because I feel it's a weaker build but otherwise I focus on AD/AS/ArmorPen in the Offensive tree. Since Kog's Ult can be a mana hog, I pick up the Mana per level talents and Improved Recall on the first tier of the Utility tree, then movement speed on the second (it's the most beneficial) and lastly Runic Affinity for the increased buff duration.

Personally I prefer Ghost and Exhaust. Some will tell you to get Flash. I don't disagree but I have had much more success with Ghost/Exhaust. Flash can get you out of a tough situation but in a 1v1, Exhaust will likely turn the tide in your favor (reduces movement speed allowing you to chase/escape, reduces damage, and the Mastery also reduces Armor/MR so more damage is dealt to the target) and can also be used to help a teammate while Ghost can be used offensively (getting to lane/team fight) or defensively (to escape). I might run Ignite over Ghost in Draft if the enemy team is running champs with crazy health regen (i.e. Vlad, Mundo, etc.). You really can take anything you want but I urge you to take Exhaust at the least.

I like to start with boots and pots for the extra mobility. I cannot stress enough how important mobility is for any ranged carry. If I'm not solo mid then I *might* consider building toward my Madred's Razor first depending on my lane partner. Note: only take mid if you don't have an AP mage or if they are completely outclassed. I rarely take Doran's Blade unless I feel I need the health but non-ranked games, this build is usually tanky enough. I rush the Madred's Razor to help get CS (creep score/last hits on minions) and push the lane to starve the enemy for gold. Next I go for either Zeal or Vampiric Scepter depending on if I need the extra sustain then finish my boots (only get Berserker's Greaves if enemy team has little/no CC, there's a reason why everybody like Mercury's Treads). If the enemy team is tanky, I rush Madred's Bloodrazor otherwise I consider turning my Razor into a Wriggle's Lantern (because wards save lives especially if you like to split push). If money is a problem I build a second Zeal before working towards Bloodthirster (the second Zeal will give more stats than making a Phantom Dancer but eats up an item slot). Note: some people prefer getting AS from items like Malady or Black Cleaver but I really like the movement speed plus you get more AS from Phantom Dancer than almost any other item (except Sword of the Divine). Phantom Dancers' biggest con (IMO) is its lack of AD. In the end it's up to you but with the Bloodrazor and 2 Phantom Dancers you almost max your Attack Speed.

The last item slot is basically a freebie. This is where you build your defensive item ( Banshee's Veil being one of the most common). If you see the enemy team stacking Armor, Last Whisper is a fantastic choice (and one reason not to get Black Cleaver). If you want a slow (and don't want to worry about red buff), you might want to build a Phage which can be built into a Frozen Mallet or Trinity Force if you don't complete your second Phantom Dancer. If you go for the Trinity Force, that still leaves you with a 6th slot for whatever item your heart desires. Personally I prefer a second Bloodthirster for the extra AD and Lifesteal (I like to split push so more damage makes towers fall faster) but it's less useful if you die frequently since you lose your stacks.

It is extremely important that you realize these are not set in stone. If you need defensive stats, build them. This build is geared towards dealing tons of damage quickly but you deal a whole lot less if you're dead.

Buy wards and enhance pots when possible.

It is important not to get tunnel vision on Madred's Bloodrazor. Often times Wriggle's Lantern is overlooked because people just worry about endgame where Wriggle's seems to fall off however they fail to realize that if they have a slow early-midgame then by the time they make their Bloodrazor, it doesn't matter. Wriggle's is a great item to help accelerate your early game allowing for sustain (lifesteal), minion farming (with the Passive proc) and the icing on the cake, free wards. I cannot stress enough how quickly this item pays for itself if used properly.

So when do you get Wriggle's Lantern? When you need the extra sustain or are ganked early and often. When you don't have the gold to purchase AD items. When you are needed to help quickly secure dragon/Baron.

Wriggle's also allows you to build a more diverse build to counter or dominate your opponent (since you won't be rushing Bloodrazor/Bloodthirster). Instead you can build a Malady or Black Cleaver or start on more defensive parts of endgame items like Phage. In essence, Wriggle's let's you keep games from actually getting to endgame.

In closing, the Wriggle's build is cheaper and aimed for a short game while Bloodrazor is more expensive and builds for endgame.

While casual players may not like it because it doesn't build into anything, more experienced players may recognize the importance of Doran's gear. For Kog'Maw it means Doran's Blade. Now you might be saying, "why Doran's? I'd rather have my Bloodthirster." It's a common reaction because of how low the stats seem. However getting one item to give you HP, AD and Lifesteal for the price of a Ruby Crystal simply cannot be ignored. Simply stacking 2-3 Doran's Blades gives a fairly decent 300 HP, 30 AD, and 9% Lifesteal for a mere 1425g (less than half the cost of a Bloodthirster). The damage from stacking Doran's might be low but it's cheap survivability.

So when do you get Doran's Blade? When you need a cheap defensive item that still gives you some offensive capabilities.

Your role is that of a Ranged AD Carry. This means you are here to down towers and destroy the enemy team. To properly fill this role you need to know when to use abilities.

Feel free to use your W and R to poke/harass since they are cheap (as long as your don't spam R that is).
Save your E for escapes or to make an enemy think twice about initiating (or to gank).
Feel free to farm unmanned lanes but always be careful (buy wards for jungle/river to prevent ganks).
Focus on last hits during the laning phase. Last hits mean more money. More money equals big items. This should be a no-brainer.
Split push when you can but don't be overly aggressive if the enemy team is MIA.
With maxed W and Madred's Bloodrazor, you can quickly cut down tanky champs so make them pay for stacking health.
Never go into a team fight first. I cannot think of a single reason you should. Just don't.
When in doubt, scout with your R. 40 mana beats facechecking the brush.
Always lead you target with your R if they are running. They'll have to juke to not get hit which means your team has a chance to catch up with them.
If you're thinking you're going to die, stand your ground and deal as much damage as you can. You'd be surprised how often you can turn around a pick up a kill with your passive. Also it forces the enemy to Recall instead of continuing to push.
Move between attacks. A novice stands there attacking while the enemy runs away. Don't be that guy. Once you build up your Phantom Dancer's, you'll have an easier time catching up with your target but it's still not advised.
Don't troll. Trolls are jerks who ruin an otherwise fun game. Be courteous because it takes more character to be graceful than to rage or insult. This one's a life tip.

While this isn't the only way to play Kog'Maw, it's the way I've had the most success with (primarily in solo/duo queue). Remember to come into team fights after they've been initiated, be aware of your minimap and surroundings, aim for last hits, call MIAs, buy wards, and remember that if you're gonna die, blow everything to make sure to take the enemy with you. Most importantly have fun.

1/3/2012 - Made changes to Runes/Masteries section and item build (pulled out hp/lvl seals, armor pen marks, flat hp quints; Rylai's removed from build)

1/20/12 - Added section "Wriggle's vs. Bloodrazor" and second build for Wriggle's. Added section "When to Doran's..."