Ordinary passport holders of most countries, including the United States, Canada, European Union countries, Russia, Japan and Australia, do not need a visa if their purpose of visit is tourism and if their stay does not exceed 30 days. 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 20 other nations, including India and China. Check the latest scoop from the Ministry of Foreign Affairs.  By law, you must carry your passport with you at all times.
Proof of onward transit, long happily ignored by Thai immigration, has for unknown reasons been zealously enforced again since 2007. (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.
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 busted overstaying by regular cops — and drug raids etc are fairly common — 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 plane
Bangkok is one of Asia's largest hubs; practically every airline that flies to Asia also flies to Bangkok, meaning competition is stiff and prices are low.
There are also international flights directly to/from Chiang Mai, Hat Yai, Ko Samui, Phuket, and Pattaya.
The national carrier is the well-regarded THAI Airways, with Bangkok Airways filling in some gaps in the nearby region. Bangkok Airways offers free internet access while you wait for boarding to start at your gate.
Many low-cost carriers serve Thailand - see Discount airlines in Asia for an up to date list.
For a full at-a-glance list of all Thai-based carriers, see the Thai airlines section (below).
 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 across the border, mostly to the southern hub of Hat Yai.
Mae Sai / Tachileik - foreigners can access this crossing from either side, and enter and/or exit either country here; no onward travel restrictions; to get to Tachileik or Kengtung from the rest of Myanmar, a domestic flight must be taken (eg from Heho).
Mae Sot / Myawaddy - 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.
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 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 , 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 railheads just across the border at Nong Khai (across the river from Vientiane) and Aranyaprathet (for Poipet, on the road to Siem Reap). There are plans to connect to both countries someday, but this is unlikely to happen anytime soon.
There are no rail services to Myanmar, but the Thai part of the infamous Burma Death Railway is still operating near Kanchanaburi.
 By ferry
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 , but no scheduled services.