Hi! This is my first champion guide so be gentle with your comments (but feel free to critique). Feel free to comment if you have any other tips. On top of that this is just how I play Mundo so it doesn't necessarily have to be the way you play him. However, I do believe that my tips and tricks section can be helpful to any mundo player so don't disregard them completely.

Most people see Dr. Mundo as a tank character or a damage per second (dps will be the abbreviation) character but still mostly just a meat shield or as someone pointed out, a potato. This build allows you to eat through tanks as well as having enough health regeneration that when sitting outside of battles for just a few seconds hundreds of hp is gained in a short amount of time. This lets you lane longer, hit harder, and essentially just be a better faster stronger opponent for your opponents to cringe when he runs in to their pathetic base. (Hint hint turret diving is ok ish as long as you throw the cleaver the get out fast)

1/16/2012
Updated to the new masteries! Sorry for taking so long!

Pros: High dps to high hp chars, lots of health regen, good in solo or team fights, can lane like a beast, and can get the lizard and golem buffs early game by themselves.

Cons: Sometimes targeted first in teamfights due to dps or misconceptions that mundo is a op char (hes not unless you play him right), a little squishy early game.

How to counter cons: in the teamfights there aint nothin to do but endure it and take the challenge head on. In the beginning when your squishy dont feel obliged to stay on the front lines, it's ok to go back to your turret and let your hp regen do its work. If necessary use ghost and exhaust to escape.

The runes are essentially there to help you eat through tanks and other high hp players. With the health regen given to you by the items and the runes you have around 170 hp regen a SECOND. if you combo that in with the cooldown reduction runes that are for your cleaver (that is your best tool trust me) and your made. The extra damage from the Gm of mights helps you do a little more dps in those team fights as well. Some people and most builds think it weird to have 3 different quints but I have found that it is too my advantage to have those 3 different ones.

So for masteries it may seem strange to put a whole bunch into that attack skill since the primary weapon for mundo is his cleaver but it throws people off and with the armor pen you also get a nice boost in dps as well letting you hit them harder faster. Most people also underestimate greed but that extra gold can really make the difference if the game is going badly. It can help you get items that you normally wouldnt be able to get giving your team and you the overall advantage and maybe a chance to turn the game around.

For items, health regen is key since it also works great with his passive. Throwing in some hp (warmogs) and magic resist (force of nature) is also just enough to usually tip the scales in a team fight with the life steal you get from starks fervor which people also dont expect due to the fact that most mundo builds dont have any life steal at all (I've gotten a lot of why does mundo have this questions in game). On top of that stark's fervor also give teammates the life steal meaning that your team will be healing with every auto attack they land. The Black cleaver is there for enemy armor reduction and a decent boost in attack power and atmas impaler is there for the defense and just ultimately making you harder to kill.

Now to do good in solo and team fights that clever of yours is a necessity as well as maochism. That clever takes awhile to get used to since its a skill shot but with its percentage based damage to there health it chows through high hp chars in a heart beat. Use it often and fast (with the hp regen you get from the build its hard to beat it out in losing hp for gaining it) Maochism boosts your dps and lets you eat through health and turrets extremely fast. With this build not only are you a great weapon against there characters you can back door as easy as twitch almost. When you use your ultimate, sadism, it makes you nearly invincible due to the massive amount of life gain. (Be wary of people with ignite it cuts your health regen in half so can be problematic if you get clipped and then have no hp to fight with. This is an example of when to use ghost and get the hell out). Some people like burning agony and use it ofter but I dont find it that helpful since it sucks hp out for very little damage. However, using it to escape crowd control effects can be quite helpful but since killing the tanks is your primary goal it doesn't make it a necessity. A hint for using the ult make opponents try to over commit to you when you fake them out with it because if they do and your health regen overtakes there damage you have a kill in the bag right there.

AAAH the summoner skills/spells, such a fun topic. For this build i suggest exhaust so you can slow down those hard to hit opponents as well as ghost in case things get hairy early game or if you need/want to chase someone down. In difference to ghost, flash would work too. In early game its easier and better to play it safe and use them defensively instead of going all out into the attack. Later in the game though if you judge correctly the use of ghost and exhaust at the right time can get you more than one kill if necessary.I will say this right now because i have seen opposing players do it, CLARITY IS USELESS TO MUNDO. He doesn't use mana so dont even bother. Also ignite isn't necessary because you don't really need the extra fire power. I suppose if clairvoyance is really your thing then you can feel free to use it but with ghost and exhaust you can escape from most ganks. I have seen a mundo use rally decently well and i suppose that the extra damage boost may be helpful but this mundo build isnt all about all out attack.

So as you may have figured out from the rest of the guide, mundo's cleaver is his bread and butter tool. With it you can gank, slow, hurt, and maim like a pro. It has a base damage that increases but the reason its so good is that it does a percentage damage equal to their current hp. It literally makes mundo someone you always have to look out for.

Another fun tip is that with the life steal and hp regen you have fighting neutral minions is a good way to heal while earning cash. Or if necessary just stand by the turret for 10 sec and you'll regen around 1600 hp. This is the key to laning for extremely long times.

Finally your ultimate, Sadism, takes a percentage of your current hp while restoring a percentage of your maximum hp at a fast rate while giving you a boosted movement speed. Using it too run away is a good use as well as making opponents commit to an opponent that they cant defeat, giving you the extra oomphf to get the kill for your team or take down a turret, maybe even get two or more kills it is that great of an ultimate that is vastly underestimated.