...READ THROUGH EVERYTHING TO MAXIMISE YOUR EFFECTIVNESS IN GAME AND LEAD YOUR TEAM TO VICTORY(APPLIES TO TEAM LEADERS AS THEY MUST KNOW THIS BETTER THEN ANYONE, GET RESEARCHING LEADERS)...

Assuming a certain level of basic skill and familiarity with tactical shooters, these Tactics should help you and your fellow Survivors get through Left 4 Dead... alive! As a start, make sure you know all of the Controls.

Above all else, no matter what team you are on, Infected or Survivors, in the end you will be depending on the people around you to survive as well as to have an overall good experience. This game requires cooperation on all sides, so be kind, treat others nicely, don't go overboard with the trash talk, do not belittle your teammates and be patient with the less experienced players. With luck, they will return the favor. If not... you might just wind up dead.

-FIGHTING THE HORDE:
Common Infected run fast, come in large numbers, and are often backed up by Special Infected. This section describes tactics you can use regardless of what your teammates are doing.

-DOORS AND CORNERS:
Always close doors when you find them. You can then safely kill Infected through the door. If the hole in the door is big enough, go ahead and use melee through the door. This will knock the Infected back, saving the door from immediate destruction, and keep you nice and safe. But also remember this can work both ways as good Smoker players will grab Survivors through the hole in the door while the rest of the group is looking in the opposite direction thinking that it's safe to be behind a door.

Closing doors behind you is also useful while you are moving through a level. The AI Director is notorious for sending Infected to you from the direction you just came, and those surprise Infected can really mess up your team quickly. Shutting doors that you just came through can slow down those sneaky Infected and mean the difference between becoming incapacitated or dying, and survival. It is especially useful to shut doors in the finale of a campaign which can make defending them a great deal easier. However, many Human-controlled Infected in Versus tend to go through the level destroying all of the closed doors. That makes it easier for the Horde to rush at and attack the team of Survivors.

When the room gets thick with Infected, the best cover in this game can be found in a corner, or failing that, a wall to your back. You can then use your melee attack to knock back Infected and shoot when you have breathing room.

Contrary to popular belief, fire extinguishers do not damage either the Survivors or the Infected. They also do not start panic events. Therefore, they should not be factored in when you are deciding where to fight a horde from.

-CHOKE POINTS:
A group of Survivors can survive the Infected horde if they use a narrow opening to draw the horde to them. This creates a slower flow of Infected, and allows the Survivors to concentrate fire-power all in one direction, making their defense more successful. Examples of effective choke points are:

A narrow hallway.
A secure room with only one entrance.
A slim rooftop with only one way to get to defending Survivors.
Effective choke point tactics include two rows of Survivors near the choke point entrance, where two Survivors are crouching up front - usually with shotguns - while the other two Survivors stand up in behind and shoot over the front Survivor's heads. Crouching Survivors can either shoot or melee, and can cover each other while their partner reloads.

Choke points can be a problem when Survivors forget that a room isn't secure from all locations - this leads to surprise zombies breaking in from another point (like through a wall or a ceiling) and overwhelming the Survivors.

Choke points are harder to find in outdoor environments, but you can always use an entrance of a building while inside to fight Infected while they try take on your group.

Other issues include Special Infected taking advantage of the tactical situation. Fortunately, Boomers and Hunters can be quickly dealt with. Smokers require Survivors to be aware of their tactics, and they must prevent their teammates from being pulled away too far, or the choke point tactics fall apart rapidly. Choke points can work against Survivors when dealing with Tanks as the Survivors are boxed in and can't use their superior speed to their advantage.

Sadly, being on top of a car, subway cabin, or another object, is of negligible value as cover in this game due to the climbing abilities of the Infected. In fact, you'll often find yourself in a situation where you are being attacked along 360°, which in most cases is fatal. However, gaining the high-ground while making sure your back is covered can buy you a good amount of time, due to the fact that it will take a few seconds for an Infected to climb up. Use melee or a well placed round or two to finish off a few of the horde.

Additionally, a group of Survivors all defending a high ground point, again using melee and fire-power can really slow down Infected's ability to disrupt a staged defense. Working together is key in high-ground defensive positions. In campaign finales especially you should find the high ground and defend it until the rescue vehicle arrives.

-SPECIAL INFECTED SEE ALSO: The Infected:

-THE BOOMER:The Survivors Useful Precautions:

Listen for the heavy breathing, gagging and burping sounds.
If you hear one, keep an eye out for defensive positions, that way you'll be ready if the Boomer blinds you.
Avoid being too bunched up with the other Survivors. There's no sense in all four of you getting puked on at once.
On expert, Boomers will often claw Survivors they have vomited on, dealing even more damage.
On expert, finding and killing the Boomer before he can vomit or explode is incredibly crucial as the Boomer bile attracts the horde, which on expert difficulty is extremely dangerous.
When the Boomer is sighted:

If the Boomer is too close, it's a good idea to hit it with your melee attack, which will push it back to some degree, then back away to a safe distance before shooting or you'll get covered in bile, even though you weren't vomited on.
Hiding behind something or running away when it's vomiting makes it harmless for some time and may even force it to rush at you clawing.
Only if the Boomer is sufficiently far away from both yourself and all other Survivors should you shoot it.
After being vomited on:

Know that a massive horde will immediately descend upon you and any others who have been slimed.
Hold your fire! Since you can't see, friendly fire is the most likely result. Use melee instead until you're sure that you won't harm you teammates.
Helping a slimed Survivor:

Stay close to the stricken Survivor.
Help melee the horde back or shoot if the blinded Survivor stays safely crouched.
Help the stricken player to safely get to a corner or high ground to protect him from the horde.
Throw a pipe bomb to distract the horde.

-BOOMER ON INFECTED TEAM:
Your job is to slow the Survivors down by attracting the horde. The Director will spawn one horde for each Survivor vomited on for a maximum of four. As a Boomer, you will need to hide and ambush Survivors since Boomers have the lowest amount of health, and emits a lot of loud burping and gurgling noises. The Boomer acts more like a support class, as they don't dish out much damage on their own. Due to this restriction, they excel when placed at the start of any coordinated attack. Their main attack attracts the horde, causing Survivor teams to panic, which creates ideal opportunities for the other Special Infected to move in and deal serious damage. Remember that you have very low health and will explode in only a few shots. If you try to rush at the Survivors in the open, they will simply kill you before you get within vomiting range.

Another weapon of the Boomer is that it explodes when it dies and covers any nearby Survivors with bile, which has the same effects as vomit and also causing them to reel back. This explosion is a good way to cover any Survivors you missed because inexperienced players have a tendency to shoot on sight instead of using a melee attack to knock it back to a safe distance. You can vomit on unsuspecting Survivors below with impunity if attacking from rooftops. Then jump down to let them shoot you in blind panic, if your health is low enough or you are high enough up you can instantly explode as you land covering any nearby Survivors! Vomiting on a person who is already covered in bile does not spawn another horde, so you should either retreat until you're ready to vomit again or to get as close to the other Survivors as you can in the hopes that they'll cause you to explode bile onto other Survivors.

Experienced Boomer players can use the knock-back effect of the explosion very effectively by knocking Survivors off high ledges normally doing large amounts of fall damage and, if high enough, will cause instant deaths. As a Boomer, don't be afraid to start clawing at Survivors if it doesn't look like you can escape.

-THE HUNTER: The Survivors When dealing with the Hunter:

There are a few rules of thumb and things you need to keep in mind:

Always have at least one buddy close at hand.
Listen for the distinctive growl the Hunter makes when he is crouched and ready to pounce. Once you hear the cues, remind the other Survivors of the Hunter's presence to keep everyone on their toes. This may sound useless, but it often prevents Survivors from taking unnecessary damage from Hunters.
Remember that the Hunter won't usually pounce as soon as he sees a Survivor, be it an AI controlled one or a player controlled one. Even the AI waits and bides its time. The cry does show you it is close by, though.
The Hunter AI on harder difficulties and some human players will often claw Survivors rather than pouncing on them. This does an extreme and deadly amount of damage. It should be avoided at all costs. Be careful as the Hunter will also sometimes try to blend into the horde and claw Survivors...