Tattoos

Tattoos have been shown to be somewhat common in the Avatar world, the most notable of them being those of the master airbenders. It seems that this type of body modification is usually for ritual or identification purposes, rather than strictly decorative.

History
It can be assumed that like traditional Chinese tattooing methods, a sharp needle-like object is soaked in some sort of ink and then repeatedly poked through the skin in order to deposit the color. This is an enormously time consuming and somewhat painful process, which is why it is seen by many cultures as a rite of passage.

Airbending masters
Once reaching master status, Airbenders are tattooed with five light blue arrows: one extending from the base of the spine to the forehead, one for each arm pointing to the hands, and similarly for the legs. The design is in emulation of the Flying Bison's natural markings, as they were the first airbenders. Unlike most master airbenders who gain their status later in life, Aang received his tattoos when he was only 12 after his invention of the air scooter. From what we have seen, only Avatars born as air nomads (like Aang and Avatar Yangchen) possess tattoos that glow. It is unclear whether or not traditional tattoos of any other Avatar would act in the same way.



Yu Yan Archers
The Yu Yan Archers bear dark red tattoos that cover their eyes and narrow over the bridge of the nose, possibly to symbolize the importance of sight in their deadly skill. Similar to those of master airbenders, the tattoos of the Yu Yan Archers identify them as part of an elite group. Because the tattoos are on their face, they are very obvious markers.

Others
As a reference to the prevalence of tattoos among prison populations, when Aang is imprisoned in the episode Avatar Day, one of the prisoners is shown with a large green serpent tattoo and states that because Aang also has tattoos he will “fit in real well”.

One of the prisoners brought to the Boiling Rock with Hakoda has an elaborate black tattoo that is possibly traditional and part of a rite of passage, or simply decorative.

The Boulder has a tattoo of a badgermole on his back, a self-proclamation of his earthbending prowess.

The third eye tattooed on Combustion Man's forehead is a sort of channel for his chi, allowing his energy to focus there and produce powerful explosions.