Ordinary passport holders of many Western and Asian countries, including most ASEAN countries, Australia, Canada, most European Union countries, Hong Kong, Japan, Russia and the United States do not need a visa if their purpose of visit is tourism. Visitors arriving by air receive 30-day permits except for citizens of Korea, Brazil, Chile and Peru who get 90 days (http://www.mfa.go.th/internet/document/ChartVisa.pdf), but effective December 15, 2008, those arriving by land are only allowed 15 days visitors from several countries, mostly nearby ones, but notably including Russians, still get 30 days on the land border due to bilateral visa exemption. Thai immigration requires visitors' passports to have a minimum of 6 months validity and at least one completely blank visa page remaining. Visa-on-arrival is available at certain entry points for passport holders of 28 other nations Bhutan, China, Cyprus, Czech, Estonia, Hungary, India, Kazakhstan, Latvia, Liechtenstein, Lithuania, Maldives, Mauritius, Oman, Poland, Russia, Saudi Arabia, Slovakia, Slovenia, Uzbekistan, Ukraine, Ethiopia, Taiwan, Bulgaria, Andorra, Malta, Romania, San Marino. Check the latest information from the Ministry of Foreign Affairs (http://www.mfa.go.th/main...). By law, you must carry your passport with you at all times (http://www.fco.gov.uk/ser...)
Those with passports from countries not widely known, including European city-states, or have problems with document forgery, should obtain a visa in advance from the nearest Thai embassy. This is true even if visa on arrival is technically permitted. There are reports of tourists being detained using valid passports not commonly presented in Thailand. In addition, ask for a business card from the person or embassy which granted the visa, so they may be contacted on arrival, if necessary. Anyone whose nationality does not have its own embassy in Bangkok, should find out which third country represents your interests there, along with local contact information.
Proof of onward transit, long happily ignored by Thai immigration, has been known to be strictly applied in some instances. Airlines, who have to pay for your return flight if immigration doesn't let you in, also check this. A print-out of an e-ticket on a budget airline is sufficient to convince the enforcers, but those planning on continuing by land may have to get a little creative. Buying a fully refundable ticket and getting it refunded once in Thailand is also an option. Land crossings, on the other hand, are a very straightforward process and no proof of onward journey required unless the border officials decide otherwise.
Overstaying in Thailand is dodgy. If you make it to Immigration and are less than 10 days over, you'll probably be allowed out with a fine of 500 baht per day. However, if for any reason you're caught overstaying by the police you'll be carted off to the notoriously unpleasant illegal immigrant holding pens and may be blacklisted from Thailand entirely. For most people it's not worth the risk: get a legal extension or do a visa run to the nearest border instead.
By train
Thailand's sole international train service links to Butterworth near Penang and Kuala Lumpur in Malaysia, continuing all the way to Singapore. Tickets are cheap even in first class sleepers, but it can be a slow ride; the 2-hour flight to Singapore will take you close to 48 hours by rail, as you have to change trains twice. The luxury option is to take the Eastern & Oriental Express (http://www.orient-express...), a refurbished super-luxury train that runs along the same route once per week, with gourmet dining, personal butler service and every other colonial perk you can think of. However, at around US$1000 one-way just from Bangkok to Butterworth, this is approximately 30 times more expensive than an ordinary first-class sleeper!
While you can't get to Laos or Cambodia by train, you can get very close, with rail terminals just across the border at Nong Khai across the river from Vientiane and Aranyaprathet for Poipet, on the road to Siem Reap. A link across to Mekong to Laos is open in March 2009, but service to Cambodia remains on the drawing board.
There are no rail services to Myanmar, but the Thai part of the infamous Burma Death Railway is still operating near Kanchanaburi.
By road
Cambodia - six international border crossings. The highway from Siem Reap and the temples of Angkor via Poipet to Aranyaprathet, once the stuff of nightmares, is now merely bad and can usually be covered in less than 3 hours.
Laos - the busiest border crossing is at the Friendship Bridge across the Mekong between Nong Khai and the Lao capital Vientiane. It's also possible to cross the Mekong at Chiang Khong / Huay Xai, Nakhon Phanom / Tha Khaek, Mukdahan / Savannakhet, and elsewhere.
Vientiane / Udon Thani - A bus service runs from the Morning Market bus station in Vientiane to the bus station in Udon Thani. The cost is 80 Baht or 22,000 Kip and the journey takes two hours. The Udon Thani airport is 30 minutes by Tuk Tuk from the bus station and is served by Thai Airways, Nok Air and Air Asia.
Malaysia and Singapore - driving up is entirely possible, although not with a rented vehicle. Main crossings with name of town on Malaysian side in brackets between Thailand and Malaysia are Padang Besar Padang Besar and Sadao Bukit Kayu Hitam in Songkhla province, Betong Pengkalan Hulu in Yala province, and Sungai Kolok Rantau Panjang in Narathiwat province. There are regular buses from Singapore to the southern hub of Hat Yai.
Myanmar:
Mae Sai / Tachileik - foreigners can access this crossing from either side, and enter and/or exit either country here; onward travel restrictions: since Oct 2009, onward travel by land to Kengtung is only possible if accompanied by an official guide 1000 BTH/day + expenses, exiting Myanmar at Tachilek is only possible for those who entered at his border crossing and were issued a 14-day Entry Permit; to get to Tachileik or Kengtung from the rest of Myanmar, a domestic flight must be taken eg from Heho.
Mae Sot / Myawaddy - This border crossing has been closed since about November 2010 and as of June 2011 there are no signs it will re-open soon. When it is open, foreigners can only access this crossing from the Thai side; neither onward travel into Myanmar ie beyond the border town nor overnight stays are possible. No visa needed; instead there's an entry stamp fee - USD10 if paid with USD notes, more 500 baht if paid with Thai currency.
Three Pagodas Pass Sangkhlaburi / Payathonzu - foreigners can only access this crossing from the Thai side; onward travel into Myanmar ie beyond the border town is not possible; entry/exit stamps are NOT issued here, and foreigners passports are held at the Myanmar checkpoint, where a fee is levied - USD10 if paid with USD notes, more 500 baht if paid with Thai currency. However, as of November 25, 2008, this crossing is temporarily closed.
Ranong / Kawthoung - foreigners can access this crossing from either side, and enter and/or exit either country here; no onward travel restrictions other than those that apply to everyone, no matter how they enter; access to/from Kawthoung is by sea Mergui/Dawei & Yangon and air Mergui & Yangon. If entering without a visa, maximum stay is 3 days / 2 nights, travel beyond Kawthoung is not permitted, and there's an entry stamp fee - USD10 if paid with USD notes, more 500 baht if paid with Thai currency.
By ship
It is possible now to travel by ferries in high season Nov-May from Phuket and island hop your way down the coast all the way to Indonesia.
This can now be done without ever touching the mainland, Phuket Thailand to Padang Indonesia.
Islands en route:
Ko Phi Phi
Ko Lanta
Ko Ngai
Ko Mook
Ko Bulon
ko Lipe
Koh lipe being the hub on the boarder between Thailand and Malaysia having a Thai immigration office.Langkawi- Malaysian immigration here.
Penang
Thai portion can be done in a day.
Ferries cross from Satun in southern Thailand to the Malaysian island of Langkawi, while over in Narathiwat province, a vehicular ferry shuttles between Tak Bai and Pengkalan Kubur, near Kota Bharu in Malaysia's Kelantan state.
There are also occasional cruises from Malaysia and Singapore to Phuket and Bangkok, the main operator being Star Cruises (http://www.starcruises.com), but no scheduled services.