Nowadays, all foreign visitors to Tibet need one or more permits. The main is the TTB (Tibet Tourism Bureau ) permit, which can be issued to you by Chinese travel agencies that handle Tibet.
For example, if you buy an expensive package tour, the TTB permit will only cost you US$6, but if you want plane/train tickets, the travel agency will inflate their cut accordingly and you'll need to fork out up to US$50-70.
In some parts of the Tibet also require an ATP (Aliens' Travel Permit), which is issued by the PSB (Public Security Bureau) in major Tibetan towns like Xigatse, Lhasa and Ali.
Bear in mind that the list of regions that require ATPs changes constantly, so enquire locally.
By road: There are 4 roads into Tibet, roughly corresponding to the cardinal directions:
• South: Nowadays, from Nepal the international border makes any sort of breaking of the rules impossible, so the only option is to book a tour with a travel agent in Kathmandu.
Since the year 2007, you need a group visa for China itself to cross the border, so don't bother applying before you get to Kathmandu. The drive from Kathmandu to Lhasa takes 5 days and is very rough, but pretty.
• East: There is no legal way to travel this road, except for an expensive organised tour;. The security is tighter than from the north.
• Norht: The road from Golmud is the easiest legal land route at present.
It's but nearly as costly as flying, if one follows all the rules, i.e. travels by bus. The landscape is very beautiful but really difficult to appreciate after the long rough ride.
• West: From Kashgar much of the way is technically off limits. But there is a steady stream of hardy travelers coming this way, usually hitching rides on trucks. The road is totally unpaved for over a thousand km with villages & water few & far between.
The main advantages of this way is that it passes by Mount Kailash & through a beautiful, very remote region inhabited by nomads.
By plane: Nowadays, you can fly to Nyingchi and also Lhasa, however flying in from a much lower altitude town puts you at high risk of altitude sickness because of the quick transition.
By train: The Qinghai-Tibet Railway, from Golmud to Lhasa began operating in July 2006. The journey all the way from Beijing takes around of 48 hours, and has a cost of 389 yuan in the cheapest hard seat class, and 1262 yuan for a soft sleeper.
Every day direct trains to Lhasa originate in Chongqing, Lanzhou, Beijing, Guangzhou, Xining and Chengdu.