During the first millennium AD the Tai peoples were loosely organised in small entities known as muang. They were heavily influenced by the more advanced cultures around them: the Khmer to the east, and the Hindu cultures of India to the west. Most of the Tai were converted to a form of Hinduism, traces of which can still be seen in Thai religious practice today. Between the 6th and 9th centuries AD Buddhism was introduced into the Tai-speaking lands, probably via Burma, and became the dominant religion. The Theravada Buddhism now practised in Thailand was introduced by missionaries from Sri Lanka in the 13th century.

Pongsawadarnnue (Chronicle of the North) is a historical record of this period. The date of first compilation is unknown, but its content strechtes from 500 AD down to early 11th century. The recent edition was compiled in early Ratanakosindra period.

The Sukhothai kingdom was an early kingdom in the area around the city Sukhothai, in north central Thailand. It existed from 1238 till 1438. The old capital, now 12 km outside of New Sukhothai in Tambon Muang Kao, is in ruins and is a Historical Park.