There are three routes to take into Tibet.

Lhasa by Air:

From within China, most travelers fly to Lhasa (Tibet's only airport) from Chengdu but it is also possible from Kunming and a few other Chinese cities.

From outside China, it is only possible from Kathmandu, Nepal. Tickets can be purchased abroad, but you can also get to Nepal or China and then book from there. All tickets must be purchased through a travel agency.

There are restrictions on travelling to Tibet. Read more about getting permits and general information in Traveling to Tibet.

Lhasa by Rail:

The Qinghai-Tibet railway was completed in July 2006 and is expected to bring in hoards of Chinese tourists. If traveling to Lhasa from within China, this is a great option as it might help you acclimatize to the altitude a bit better.

You can take the train all the way from Beijing to Lhasa with a stop in Xi'an to see the Terracotta Warriors.

Overland to Lhasa:

While there are a number of routes into Tibet, officially only two allow foreign travelers.

• The first is via Nepal. Many travelers book overland tours via the Friendship Highway that runs from Kathmandu to Lhasa. The 920km (570 miles) trip is spectacular although don't expect a shower along the way. It takes three to five days with stops at sights along the way.
• The second option is via Golmud in Qinghai province. The terrain is desolate so it's not recommended unless you happen to find yourself in Golmud, wishing to be in Lhasa. If this is the case, you may have a better time on the train (see above).