Tibet is, as you're probably aware, controversially under the control of China. They call it the Tibet autonomous zone and require all travellers to have a permit to go there.
The Tibetan tourism board re-opened the permit process on April 1st after a 2 month hiatus.
You will need a standard visitor visa for China. Don't mention Tibet as one of your destinations in the application process - they get very sensitive about it.
Independent travel in Tibet is not permitted and you must be a part of an authorised agent's tour (even though they can organise a "solo" tour group).
If you intend to travel to further restricted parts of Tibet you'll need even more permits.
The entry permit can be issued in 3 days, but allow at least 2 weeks. You'll need the permit before you can board a train or plane for Tibet. over 7 years ago