Visas are not required by all nationals from UK, Australia, USA, Canada and EU countries for touristic stays of up to 30 days, provided they hold valid passports, sufficient funds and confirmed tickets to leave Thailand. Please note that if you arrive by land into Thailand then you only get 15 days stay and not 30 days as arriving by air.
All visitors are required to have a passport valid for six months from entry into the country.
Visa Note
(a) The total duration of stay in Thailand for persons who enter Thailand without a visa cannot exceed 90 days during any six month period, counting from the date of first entry. (b) If you intend on staying or working in Thailand for longer than 30 days you must obtain an extension of stay or a valid visa.
Types of Visa and Cost
Single-entry tourist visa: £28; double-entry tourist visa: £56.
Validity
Single-entry: three months. Multiple-entry: six months.
Applications to:
Consulate (or consular section at embassy or high commission).
Extension of Stay: Those who wish to stay longer or may wish to change their status of visa must file an application for permission at the Office of Immigration Bureau located on Soi Suan Plu, off South Sathorn Road, Bangkok 10120 , Tel (662) 287-3101-10 (or at http://www.police.go.th/thaiimb/frconten.htm). The extension of stay as well as the change of certain type of visa is solely at the discretion of the Immigration officer.
US citizens who do not wish to go through the trouble of dealing with the red tape themselves can count on the professional services of Travel Visa Pro to speed things up: Apply for Thailand Visa Online (service starts from $39). If you need to renew, add pages, change name, or just get a new US passport, you can Apply for a New US Passport here.