Straight from US Embassy site:
ENTRY/EXIT REQUIREMENTS: If you are a U.S. citizen tourist staying for fewer than 30 days, you do not require a visa, but your passport must have at least six months validity remaining. You may be asked to show an onward/return ticket. If you enter Thailand by air without a visa, you are allowed to stay in Thailand for 30 days per visit. If you enter Thailand by land without a visa, you are allowed to stay in Thailand for 15 days per visit. If you enter Thailand without a visa, you cannot remain in Thailand for more than 90 days during any six-month period, counting from the date of first entry. After 90 days, you must apply for a new visa at a Thai embassy outside of the country. You must pay a Passenger Service Charge in Thai baht when you departi from any of Thailand's international airports. This charge is now included in airline ticket prices at Bangkok's main airport, Suvarnabhumi International Airport.