What is the Avatar?

The Avatar is the incarnation of the Spirit of the Planet in human form.

There is only one Avatar, who reincarnates himself through the centuries. Each time the Avatar dies it is reborn into the next nation. This successive cycle mirrors the order of the seasons:
Water Tribes - Winter
Earth Kingdom - Spring
Fire Nation - Summer
Air Nomads - Autumn
The cycle provides a natural balance between the elements. This way, no single nation can grow stronger than another.

Upon the death of an Avatar, Bending masters from the successive nation conduct a search to find the child reincarnate. (The successor is expected to show signs of continuity with the one who died, such as being born within a week of the death.) These Benders carefully train the new Avatar to prepare him for his high position. First, he becomes proficient with his native element. Then, when the Avatar reaches maturity, he travels the world and studies with Masters from the other three Bending disciplines. Upon completion of his training, the Avatar possesses sole control of all four elements.

And so, the nations work to nurture the Avatar, whose responsibility it is to keep Water, Earth, Fire, and Air in harmony with each other and the world.