Some of the army lists include characters who are allowed to use psychic powers. These characters are often referred to as psykers. Psychic powers vary from race to race, so the effect of each power and when it may be used is included in the special rules for the psyker in the army list. The rules below tell you how to find out if the psyker can use their powers, and what can happen to him if things go wrong ...
It requires intense concentration and inner calm for a psyker to use their power, which is not always easy to attain in the midst of a firefight! In order to use one of their powers the psyker must make a Psychic test by rolling equal to or under their Leadership on 2D6. If they fail the psyker's concentration has been broken and they cannot use the power that turn. If they pass, the power may be used as described in the special rules for the psyker in their army list. A psyker may not attempt to use more than one power per turn.
Psychic attacks are subject to the usual Shooting rules, so the psyker must be able to see the target, and all attacks must be directed against a single target unit, etc. This is because even telepathic psykers find that in the heat of combat it is hard to pinpoint the mind of an opponent with any clarity: generally, they all blur and merge in a confusing swirl.