As a general rule, any foreigner travelling to Thailand is required to obtain a visa. There are a number of different types of visas available depending on the length and purpose of the visit.
However, in the case of tourism, there are certain instances where travellers may be exempt from visa application. If you are a passport holder of one of the following countries listed below and staying in Thailand for less than 30 days, then you are not required to obtain a visa:-
Australia, Austria, Bahrain, Belgium, Brazil, Brunei, Canada, Czech Republic, Denmark, Finland, France, Germany, Greece, Hong Kong, Hungary, Iceland, Indonesia, Ireland, Israel, Italy, Japan, Kuwait, Liechtenstein, Luxembourg, Malaysia, Monaco, Netherlands, New Zealand, Norway, Oman, Peru, Philippines, Poland, Portugal, Qatar, Singapore, Slovak Republic, Slovenia, South Africa, South Korean Republic, Spain, Sweden, Switzerland, Turkey, United Arab Emirates, United Kingdom, Untied States of America, Vietnam.
If you are entering Thailand under visa exemption then you must provide a passport with at least six months validity as well as proof of onwards travel to leave Thailand within 30 days of arrival, otherwise a tourist visa must be obtained prior to arrival. In addition to this, you must prove adequate finance for the duration of your stay (travellers cheques or cash equivalent to 10,000 Baht per person).
In some instances, a tourist visa must be completed on arrival, which allows the traveller to stay within Thailand for 15 days. This applies to visitors from the following countries:-
Andorra, Bhutan, Bulgaria, China, Cyprus, Estonia, Ethiopia, India, Kazakhstan, Latvia, Lithuania, Maldives, Malta, Mauritius, Romania, Russia, San Marino, Saudi Arabia, Taiwan, Ukraine, Uzbekistan.
Any travellers with passports issued in countries which do not appear on the above lists need to obtain a visa before arriving in Thailand . This form must also be used if visitors wish to stay longer than their allotted time periods. For those wishing to visit Thailand for purposes other than tourism, there are alternative visas.
Once in the country, it is quite often not possible to extend your stay in Thailand. Extensions are only granted in exceptional circumstances and are at the discretion of the Thai immigration officers.
All visa information outlined above is subject to changes by the Thai Ministry of Foreign Affairs and as such, it is your responsibility to ensure you have the correct visa & documentation to enable you to entry Thailand.