In Tibetan Buddhism, the successive Dalai Lamas form a tulku lineage of Gelugpa leaders which trace back to 1391. Tibetan Buddhists believe the Dalai Lama to be one of innumerable incarnations of Avalokitesvara ("Chenrezig" [spyan ras gzigs] in Tibetan), the bodhisattva of compassion. Between the 17th century and 1959, the Dalai Lama was the head of the Tibetan government, controlling a large portion of the country from the capital Lhasa. The Dalai Lama is the supreme head of Tibetan Buddhism, and the leaders of all four schools consider the Dalai Lama to be the highest lama of the Tibetan traditions. He is often granted the style "His Holiness" (or HH) before his title.
From the Wikipedia entry on the Dalai Lama. See more.
Order design in: