Upon arrival in Thailand, foreign citizens must hold a valid passport with at least six months validity, a valid Thai visa, and proof on onward travel.
However, visitors from ASEAN or citizens from some European, Commonwealth, and North American countries are not required to have a Thai visa for visits less than 30 days. They will be granted access through Thailand immigration with a 30-day Thai visa waiver, provided upon arrival. After such time, a Thai visa extension from an immigration office or a Thailand visa from an embassy or consulate outside of Thailand is required.
Immigration policies are subject to change: Check with a Thai Embassy or The Ministry of Foreign Affairs for the latest information. http://www.mfa.go.th/main/en/services/4908
Note that proof of Yellow-Fever vaccination is required upon arrival if you are travelling from yellow fever infected country.