If your Xbox 360 isn't connected to Xbox Live, hang your head in shame and moan like a flesh-starved zombie. You offliners are missing the multiplayer event of the year. Whether you assume the role of a human fighting for survival within the wastes of a dead city or the zombie who eyes this walking meat buffet as a tasty appetizer, Left 4 Dead's online battles will consume your soul and bathe you in the blood of one of video games' most imaginative, visceral, and indecently impactful experiences.

When you fire the game up, three additional players join you. If you select cooperative play, you and your teammates must blaze a trail through AI-controlled zombie forces to a safe house  where weapons, ammo, and precious health packs await. If you select competitive play, the goal for your team remains the same, but a second team of humans now controls the elite zombies (which I'll detail later). The goal for each team is simple: survive, or on the contrary, kill.

The gameplay for the two factions is completely different, but at the same time, unified by the game's strongest component - teamwork. If you wander off for just one second, you could endanger the lives of everyone on your team, or if you are the zombies, allow the humans to reach the safe house. Even if you've played through a particular level 100 times, it's impossible to predict when and how the zombies will attack your squad. The level's geometry never changes, but the zombie spawns do. In one playthrough, a street may be calm and quiet. In another, it could look like a rolling river of undead bodies. Thus, it is imperative that you stick together, and better yet, coordinate your tactics.

Survivors can only carry two weapons: a default pistol and your choice of an assault rifle, shotgun, or sniper rifle. The sniper rifle may seem like an odd weapon to use against zombies hordes that are faster than Carl Lewis, but its lightning quick reloads makes it reliable. The control mechanics are mapped strangely (with reload on B, and no iron sight), but perfectly fit the action. Targeting (with a little aim assist) is spot on, pipe bomb grenades are easy to place, and the melee gun bash is a highly effective way to keep zombies from gnawing on your bones.

If you are playing as the undead axis, each respawn places you in the body of either a smoker (who grabs survivors with his tongue from afar and slashes them when they get near), a hunter (who pounces onto survivors and claws them to death), a boomer (a ball of lard that moves slowly and blinds survivors with his pukey belch), or, on rare occasions, a tank (a Hulk-like abomination that can smack humans into next week and take entire magazines of bullets). The tank's movements are sluggish, but each baddie is a riot to play, especially when your team strategizes to launch full-on assaults against the survivors.

After playing just one match, you'll think twice before mocking the cheesy dialogue found in b-rate zombie movies again. Given the intensity of each fight, you'll find yourself screaming ''get it off me, man!'', ''eat lead, fleshbag!'' or whatever groan-inducing one-liner your brain can produce after a hard fought victory. If you don't say anything, there's a chance a smoker may make off with you, or a teammate may not get the ammo he or she needs to hold off a swarm.

While they deliver a thrill a second, both the competitive and multiplayer modes are limited in content. The game only consists of four levels, each lasting about an hour. The amazing level designs will remind you of every zombie movie ever made. All of them conclude in style as well, with survivors barring themselves in a building as zombies tear through the doors and windows, and also a rooftop showdown that concludes with undead bodies pouring off its sides with the consistency of rain. The lack of levels is a major letdown, and the fact that each level recycles the same five enemy types doesn't help matters. Let's just cross our fingers and hope Valve quickly delivers downloadable content and/or expansion packs.

I've neglected to talk about single player, which the game lists as the third option on its title screen. There is no story to soak in, and sadly, it's just not the same game without buddies at your side. If anything, this mode is worth a look just to witness Valve's amazing teammate AI. They are almost too good, often shooting a little too quickly and taking down elite zombies before you see them (which sucks for multiplayer matches without eight players). The split-screen co-op is fun, but results in a drop in graphical fidelity.

If you are even thinking about playing this game, pony up for the Xbox Live Gold membership. It's just as necessary as the controller. While light on content, Left 4 Dead needs to be in everyone's library, not only because it innovates in ways that will shape the future of gaming, but also because it's so *** fun.