Less than 30 days
Citizens of the majority of American and European countries can enter Thailand without a visa for stays of under 30 days. You need a passport with at least 6 months' validity. These countries include:
Australia, Austria, Belgium, Bahrain, Brunei, Canada, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hong Kong, Hungary, Iceland, Indonesia, Ireland, Israel, Italy, Japan, Kuwait, Luxemburg, Malaysia, Netherlands, New Zealand, Norway, Oman, Philippines, Poland, Portugal, Qatar, Singapore, Slovakia, Slovenia, Spain, South Africa, Switzerland, Sweden, Turkey, U.K., U.S.A, UAE and Vietnam.
Countries that need a visa
Among the countries that need a visa for stays of over 15 days are: Andorra, Bulgaria, Bhutan, China, Cypris, Ethiopia, India, Kazakhstan, Latvia, Lithuania, Maldives, Malta, Mauritania, Romania, San Marino, Saudi Arabia, Taiwan, Ukraine and Uzbekistan.
Check Your Status
You can check in greater detail on the official Thai government website