Infiltrate is the ability of certain troop types, often the Elites of a given army, to sneak onto the battlefield ahead of the main force. They set up in forward positions and offer fire support, or launch raids into the enemy lines. An infiltrating unit can cause all sorts of mayhem if used properly

To represent this ability, troops that Infiltrate set up after the rest of your army. They can deploy anywhere on the board that is 18" or more from the enemy, or 12" if no enemy can draw line of sight to them.
 

Being able to deploy them after your opponent's entire army is on the table is the most beneficial part of the Infiltrate ability. They can be used to reinforce a weak point in your deployment, or try and exploit a weak point in your opponent's deployment. Infiltrators are very often lightly armored and should be ideally deployed in a protected position. The first thing to consider when determining where to deploy an infiltrating unit is: What's it's job?
 

Infiltrators equipped for fire support are usually armed with long range weapons and placed in a spot with good fields of fire. By deploying them last, after your opponent's army is on the board, you should be able to get them into a prime spot with line of sight to several enemy units.

Units with sniper weapons, like Eldar Rangers, are especially useful in a fire support role. Most players HATE pinning weapons, and will often alter their route of advance rather than face down unit of snipers.
 

Just like the "Bullet Magnet" tactic I talked about in the previous section, the threat of snipers can be far more effective than their actual performance. A well placed squad with sniper weapons can disrupt your opponent's plans without firing a single shot or being anywhere near the enemy. I've seen it happen. That's psychological warfare at it's best.
 

Close Support Infiltrators armed with short to medium range, high-strength weapons can pose a big threat to vehicles and infantry alike. Imperial Guard Veterans with meltaguns or plasma weapons fall into this category nicely.

They can be sitting ducks if unsupported, so it's not usually wise to deploy them too far away from your line, unless you're using them as suicide troops to disrupt your opponent's plans. This can be a risky tactic, but I've seen Infiltrators with special weapons disrupt an entire flank and throw a wrench in the other player's plan.
 

Units like Genestealers can be a nightmare if used properly. Close combat infiltrators dropped into a blind spot in your enemy's deployment zone will REALLY mess up his plans.

However, doing something like this will mean the unit is unsupported by the rest of your army. The Infiltrators will very likely get wiped out, no player worth his salt will allow a unit like this to live for long. But if you can cripple a unit and cause a turn or two of disruption, they can buy time for the rest of your army to get into position.
 

You will have to consider if it's worth it. Will they be able to cause enough damage and mayhem to make them worth the investment? Units like this MUST be deployed out of line of sight, as your opponent will be dedicating a lot of firepower to take them out. Cover saves alone will not cut it.
 

This is a catch-all category that covers generalist units like Ork Kommandos and Space Marine Scouts (without sniper rifles). These units can mount a pretty effective assault by having a tooled up squad leader. They can also play the Close Support role pretty well too depending on what weapons they are armed with.

They are not quite as effective at any one role as a specialist squad, but their ability to fill a variety of tactical roles makes them a good choice. Their flexibility means you can watch where your opponent deploys, get an idea of what he's got planned, and then place your infiltrators where you think they will be most effective.
 

Aside from the unit-specific battlefield roles listed above, there are a few other tactics that infiltrators excel at. These tactics are based on using the Infiltrate ability, regardless of the role the unit was designed for.

Counter-Infiltration: If your opponent is using infiltrators as well, you can use your infiltrators to minimize their effectiveness. For example, both armies have been deployed and you notice there's a nice patch of area terrain that your opponent can use to get his infiltrators close to your line. If you win the roll to deploy infiltrators first, stick your guys into that terrain to keep your opponent from doing it to you. If you don't win the roll and your opponent places his infiltrators into that terrain, you should be able to get your guys into a position fairly close to try and counter them.

Objective Camping: This tactic is especially effective in missions with Loot Counters or Table Quarters. Just by parking your infiltrators in a table quarter (or near an objective), you have essentially captured victory points before the game has even started. Your opponent now has to devote resources to counter the infiltrating unit, or concede those points.

Grabbing objectives in this way is a very powerful ability of infiltrators and one I have used to great effect many times. It gives you the initiative in the game and puts your opponent on his back foot, forcing him to react to what you're doing, instead of the other way around.
 

Infiltrators have many uses, but have to be used carefully or they will get wiped out fairly easily. They are not super-units by any means (though a tooled up squad of Chaos Chosen comes close). Knowing when, where, and most importantly how to use an infiltrating unit is the key to their effectiveness. Like anything else, it takes some practice.