Tibet was a former nation of central Asia occupied by China.
The Fifth Doctor travelled to Tibet in 1917, where he battled the Great Old Ones who attempted to be unleashed on Earth via his companion Erimem. (AUDIO: The Roof of the World)
The Second Doctor had previously visited Tibet in 1935 with Jamie McCrimmon and Victoria Waterfield, encountering the Great Intelligence for the first time. He also met a group of monks at the Det-Sen Monastery whom he helped to fight the Yeti of the Himalayas. (TV: The Abominable Snowmen) Later, the Third Doctor proved to be familiar with the customs of the country with K'anpo Rimpoche. (TV: Planet of the Spiders)
Victoria returned there decades later. (PROSE: Downtime)
K'anpo Rimpoche lived in Tibet for a time, learning the customs, the Tibetan language, and religion of that people. (TV: Planet of the Spiders)
The Eleventh Doctor, Amy Pond and Rory Williams visited Tibet while trying to get to Rio de Janeiro. (PROSE: The Glamour Chase)