U.S. citizens arriving by air need only a valid passport, not a prearranged visa, to visit Thailand for less than 30 days. Technically, travelers need an outgoing ticket and "adequate finances" for the duration of their Thailand stay to receive a 30-day stamp upon entry, but authorities in Bangkok rarely check your finances, unless you look like the sort who might sponge off Thai society. They do occasionally ask to see an outbound ticket. Scrutiny is inconsistent; authorities periodically crack down on long-term tourists who try to hang out in Thailand indefinitely by making monthly "visa runs" across international borders. Tourists who arrive in Thailand by land from a neighboring country are now granted only a 14-day visa. As of this writing, tourists are not allowed to spend more than 90 days of any six-month period in Thailand, and immigration officials sometimes opt to count days and stamps in your passport.
If for whatever reason you are traveling to Thailand on a one-way ticket, airline officials might ask you to sign a waiver before allowing you to board, relieving them of responsibility should you be turned away at immigration.
If you want to stay longer than one month, you can apply for a 60-day tourist visa through a Royal Thai embassy. The embassy in Washington, D.C., charges about $40 for this visa, and you'll need to show them a round-trip ticket and a current bank statement to prove you can afford the trip. Be sure to apply for the correct number of entries; for example, if you're going to Laos for a few days in the middle of your stay, you'll need to apply for two Thailand entries.
Tourist visas can also be extended one month at a time once you're in Thailand. You must apply in person at a Thai immigration office; expect the process to take a day. You application will be granted at the discretion of the immigration office where you apply.
If you overstay your visa by a day or two, you'll have to pay a B500 fine for each day overstayed when you leave the country. Recently, immigration officials have reportedly started jailing foreigners who overstay by more than six weeks.
U.S. Department of State (877/487–2778. travel.state.gov/passport.)
Bangkok Immigration (Soi Suan Phlu, South Sathorn Rd., Bangkok, 10120. 02/141–9889.)
Chiang Mai Immigration (71 Airport Road, Chiang Mai, 50200. 053/201–755.)
Royal Thai Embassy in Washington, D.C. (202/944–3600. www.thaiembdc.org.)