Concealment

If you can’t get a good look at your target, it has concealment from you, which means your attack rolls take a penalty against that target. You might be fighting in an area of dim light, in an area filled with smoke or mist, or among terrain features that get in the way of your vision, such as foliage.

• Obscured squares
  • Lightly obscured: Squares of dim light, foliage, fog, smoke, heavy falling snow, or rain are lightly obscured.
  • Heavily obscured: Squares of heavy fog, heavy smoke, or heavy foliage are heavily obscured.
  • Totally obscured: Squares of darkness are totally obscured.

Effects that cause concealment obscure vision without preventing attacks.

• Concealment
  • Partial concealment: −2 penalty to attack rolls. The target is in a lightly obscured square or in a heavily obscured square but adjacent to you. In earlier sources, partial concealment was called "concealment".
  • Total concealment: −5 penalty to attack rolls. You can’t see the target. The target is invisible, in a totally obscured square, or in a heavily obscured square and not adjacent to you.
  • Melee attacks and ranged attacks only: Attack penalties from concealment apply only to the targets of melee or ranged attacks.

Part of the challenge of attacking a target you can’t see is knowing where to direct your attack. You have to choose a square to attack, and the target might not even be in that square. A variety of powers and other effects can render you invisible, effectively giving you total concealment.