The direction of the front of the model's base is assumed to be the actual direction faced by the creature it represents. Common sense dictates that a model cannot shoot at something it cannot see. To represent this, a target must lie within a 90° arc projected from the front of the shooter. Where models are on square bases, this can be imagined easily by projecting a line through the corners. This is called the line of sight and it determines which opponents the model can shoot at or charge.
A shooter's line of sight may be blocked if there is anything between him and his intended target which obscures his line of sight. Thus, interposing models or scenery may block a model's line of sight to a target. Because of this, except as noted later, only models in the front rank of a unit are able to shoot, as those behind will not be able to see past their friends.
Imagine a real battlefield with its contours, morning mists and haze of dust. Picture the woods and hedges that obscure vision, the sudden fall of ground that hides the enemy, and the distances that blur friend with foe. Towering over our miniature battlefield we are unaware of all this, but the troops represented by our models would not be so lucky. Just as their real life counterparts cannot see through hills or hedges, we must assume that our models cannot see behind corresponding scenic features.
As it is impossible for us to say exactly what everyone's scenery looks like, it is not practical to be definitive about which kinds of terrain block line of sight. You must be prepared to use your own judgement within the following guidelines.
Perhaps the easiest way of checking what a model can see is to get down over the table and take a model's eye view, but be reasonable about this, as in reality it would be much more difficult to see enemy troops than over a perfectly flat, mist-free gaming table.
Hills, large boulders and buildings obscure sight over level ground. If an interposing hill or house completely obscures a line of sight to a model on the other side of it, you may not see through it and so cannot shoot at the model on the other side.
Hedges and walls block line of sight over level ground. However, a model placed directly behind and touching an obstacle is assumed to be able to see and shoot over the obstacle, with head and shoulders clearly visible. Such a model can shoot but also be seen and shot at.
Woods block line of sight if the shooter and the target lie on either side of the wood. It is only possible to see through up to 2" of woodland, so if a model inside a wood is within 2" of the edge, he can see out and shoot and he can also be seen and shot at (a -1 penalty applies). If it is further than 2" inside the wood, a model neither be seen by models outside nor can he see them. If both target and shooter are inside the wood then the missile range is reduced to the farthest they can see - which is 2".
Troops, either friendly or unfriendly, block line of sight. It is not possible to shoot directly through one model to hit another. This does not apply if a target behind normal-sized models (such as Men or Orcs) is defined as a large target. Snotlings or Goblins can't block line of sight to a Giant, for example! This works vice versa - a Dragonrider, for example, can shoot at targets over interposing friendly models which are not large.
Note that this does not allow large creatures to charge through any interposing models!