Doppelgangers are monstrous humanoids, identified primarily by their ability to change their shape and appearance to mimic almost any humanoid creature. They have long, gangly limbs which betray their above-average strength and agility, and pale gray skin. Their eyes resemble those of an octopus, yellow with slitted pupils, and they have no hair on any part of their body. Their facial features appear to be half-developed.
Doppelgangers are cunning and patient, and will wait as long as it takes for an opportunity to present itself. They have the ability to Detect Thoughts, which helps them to mimic their target almost flawlessly by imitating their mannerisms and discovering information that only their target would know. Because of this, they are feared and regarded as natural spies and assassins.
Doppelganger families tend not to be close; usually a male and female meet, engage in casual sex, then leave one another, with the female being forced to single-handedly rear any resulting newborns—who will leave their mother upon reaching maturity. Some Doppelgangers, however, form tightly knit family clusters, while others will mate with humanoids, taking malicious delight in using their non-Doppelganger spouse as a cover for their inhumanity. Children born of such unions have a chance of being a half-Doppelganger, while decendants can find them selves as a Doppleganger Bloodline.
In the Eberron campaign setting, there is a race descended from half-human/half-Doppelgangers, known as Changelings. They breed true, like Doppelgangers do, but have more distinct facial features in their natural form. They can only change their physical appearance, not including their clothes, and they cannot Detect Thoughts. Their favored class is Rogue.
A subspecies of Doppelganger can be found in the Demiplane of Dread, Ravenloft. These dread Doppelgangers, like many Ravenloftian creatures, are a twisted rendition of the original. Dread Doppelgangers are capable of mimicking with 100% efficiency as well as generating natural illusions via their "Glamer" ability, but in the process have become sexless parasites, dependent on their ability to integrate with society to survive. While normal Doppelgangers have their own culture and have no real need to mingle with humanoids, Dread Doppelgangers can only reproduce by mating with humanoids (usually doing so as males, as a "pregnant" dread doppelgänger cannot change forms until after giving birth) and have effectively lost their own culture.
Dread Doppelgangers have one major accomplishment over their natural kin: their own country. The Ravenloft domain of Paridon is the birthplace of the dread Doppelganger, and as such is crawling with the vile things. The Darklord of the domain, Sodo the Flickerflame, directs and organizes a vile ritual called the Blood Rite, which takes place once every thirteen years and is used by Sodo to give himself effective immortality. This ritual is conducted by a chosen dread Doppelganger, and is referred to by the Paridoners as "The Bloody Jack Killings" because for many years Paridon thought the killings were the work of a madman.
Doppelgangers in 3rd Edition
In the 3rd edition of Dungeons & Dragons, Doppelgangers were classified as shapechangers. When this creature type was made obsolete in 3.5 Edition, their type was changed to Monstrous Humanoid. If used as a PC race, their favored class is Rogue.
Doppelgangers in 4th Edition
Early reports from the D&D Experience convention say that Changelings and Doppelgangers are one and the same race, which will be playable straight out of the Monster Manual. This either suggests that the Doppelganger's dramatic shapechanging ability has been toned back to what the Changeling was capable of before, or that the Changeling's natural Alter Self ability has been tuned up.