Increasing the game's sense of realism is the A.I. enemy and ally soldiers, who move in units, take cover, shoot out from around corners, throw substantial amounts of grenades, and always look for ways to flank their targets. Players are rewarded for staying and working with their unit rather than going Rambo, especially when turning the difficulty level up to Veteran.

Overall, the single player campaign alone is worthy of a $60 price tag. It's not perfect — the grenade indicator system is a bit inaccurate, and some of the weapons feel underpowered — but after you silently knife your first Russian ultranationalist, those things won't matter.

From the onset of development (after the release of Call of Duty 2) Infinity Ward devoted a full team of programmers to COD4's multiplayer interface, and it shows. The presentation and depth of online play is unmatched by any other first-person shooter that has come out this year.

At the hub of the multiplayer experience is the ranking system. As players obtain kills, win matches, capture flags, and so on through their online career, they will collect experience points. When a certain amount of experience points has been earned, the player will get promoted to the next rank.

On top of giving the multiplayer a very unified feel, this promotion system holds the key to unlocking new weapons, new weapon attachments, new camouflage patterns, new perks, and new challenges.

Perks are abilities that players assign to their online avatar to aid them in battle. For example, the "Last Stand" perk lets the player pull out their pistol after they have been gunned down, and fire off a few rounds before they give in. Other perks allow for increased bullet damage, increased health, the ability to carry more grenades (or other explosives), and the ability to jam the enemy radar. Perks are a great way for players to customize their time online in a way that matches their play style.

Adding even more spice to the multiplayer game are the challenges. Challenges are optional tasks (like shooting down an enemy helicopter or falling to your death from a certain height), which, if completed, net the player a healthy amount of experience points and other unlockable items. While they aren't required to progress through the leveling system, they certainly make the journey to rank 55 shorter.

Outside of perks and challenges, the online gameplay itself is fast and frantic, but allows for and encourages a high degree of team cooperation. The best example of this is with the kill streak rewards. For every three, five, and seven kills a player achieves without dying, a reward is given. Three kills allow the player to call in a UAV (Unmanned Aerial Vehicle) which marks all unfriendly combatants on the radar, five kills get the player an air strike, and seven kills net the player his very own friendly helicopter. The use of teamwork when dealing out these rewards greatly increases the chances of their success. For instance, if one team member uses a UAV, his ally will know exactly where the best place to call down an air strike is.