Perception

Perception
Perception replaces both Spot and Listen. Under most circumstances, the skill will take the place of both rolls; however, if something can be perceived only by sight and the character is blind or otherwise visually impaired, then the DM is within his rights not to allow a Perception check.

Perception DCs are equal to the Spot and Listen DCs given in the Player's Handbook. Perception is also what is rolled to oppose Stealth rolls.

Perception vs. Stealth
Characters are entitled to reflexive Perception checks whenever they first encounter a Stealthed character, and if that Stealthed character takes certain actions (such as attacking from concealment, or leaving concealment to approach the character on their blind sides). Characters who are deafened are not entitled to a check on approach. Blindness, deafness, and environmental variables also affect Perception, as detailed here:

A character who is both blinded and deafened cannot make Perception checks at all, unless he possesses some manner of extraordinary sense (scent or the like).

A character who can see automatically notices a Stealthed character who does not have concealment and who is in one of his facing squares; see Stealth for further details.

Scent
Creatures with the Scent special ability are entitled to special Perception checks to notice creatures, including Stealthed creatures. The DC of this check is 10, and increases by +1 per 10 feet, to a maximum range of 50 feet. Treat downwind targets as being twice the distance, and upwind targets as being half the distance.

A successful Scent check against a Stealthed target allows the creature to pinpoint the Stealthed target, thereby breaking Stealth. The creature may inform his allies on its next action as usual.

Overpowering scents, such as a skunks' musk or a troglodyte's stench, are detectable at triple the normal range; however, they overwhelm lesser scents in the area. A creature in the radius of an overwhelming scent is not able to pinpoint lesser scents within that area.

Change
Making a Perception check is a standard action. Reflexive Perception checks are always treated as though the character is taking 10.

Synergy
A character with 5 or more ranks in Perception gains a +2 synergy bonus to Sense Motive. A character with 5 or more ranks in Survival gains a +2 synergy bonus to Perception checks in natural environments.