Travel into Tibet is tightly controlled. Foreign visitors need a Tibet Travel Permit (TTP) in addition to a Chinese visa — apply through a licensed Chinese tour operator at least 20 days before arrival, since the permit must be in hand before you can board a flight or train into the region.
By air, Lhasa Gonggar Airport has direct flights from Chengdu (the main gateway, around 2 hours), Beijing, Shanghai, Xi'an, Kunming, Chongqing and Kathmandu. Flying is the easiest option but gives you no altitude acclimatisation — plan to rest for two days on arrival. By train, the Qinghai–Tibet Railway from Xining to Lhasa is one of the great train journeys of the world: 22 hours across the plateau, crossing passes over 5,000 m with piped oxygen in the carriages. Trains also run through from Beijing (40 hours), Shanghai (47 hours) and Chengdu (36 hours). By road, overland access from Nepal (via Zhangmu and Gyirong) is occasionally possible but has been closed for long periods since the 2015 earthquake.