Sukhothai Period 
Although animistic beliefs remained potent in Sukhothai, King Ramkhamhaeng and his successors were all devout Buddhist rulers who made merit on a large scale. The major cities of the Sukhothai kingdom were, therefore, full of monasteries, many of which were splendid examples of Thai Buddhist architecture. Sukhothai adopted the Ceylonese school of Theravada Buddhism, beginning with King Ramkhamhaeng's invitation to Ceylonese monks to come over and purify Buddhism in his kingdom. This Ceylonese influence manifested itself not only in matters of doctrine but also in religious architecture. The bell-shaped stupa, so familiar in Thai religious architecture, was derived from Ceylonese models. Sukhothai style Buddha images are distinctive for their elegance and stylized beauty, and Sukhothai's artists introduced the graceful form of the "walking Buddha" into Buddhist sculpture.Sukhothai's cultural importance in Thai history also derives from the fact that the Thai script evolved into a definite form during King Ramkhamhaeng's time, taking as its models the ancient Mon and Khmer scripts. Indeed, this remarkable king is credited with having invented the Thai script.
King Si Inthrathit and King Ramkhamhaeng were both warrior kings and extended their territories far and wide. Their successors, however, could not maintain such a far-flung empire. Some of these later kings were more remarkable for their religious piety and extensive building activities than for their warlike exploits. An example of this type of Buddhist ruler was King Mahathammaracha Lithai, believed to have been the compiler of the Tribhumikatha, an early Thai book on the Buddhist universe or cosmos. The political decline of Sukhothai was, however, not wholly owing to deficiencies in leadership. Rather it resulted from the emergence of strong Thai states further south, whose political and economic power began to challenge Sukhothai during the latter half of the 14th century. These southern states, especially Ayutthaya, were able to deny Sukhothai access to the area.
The Sukhothai kingdom did not die a quick death. Its decline lasted from the mid-14th until the 15th century. In 1378, the Ayutthaya King Borommaracha I subdued Sukhothai's frontier city of Chakangrao [Kamphaengphet], and henceforth Sukhothai became a tributary state of Ayutthaya. Sukhothai later attempted to break loose from Ayutthaya but with no real success, until in the 15th century it was incorporated into the Ayutthaya kingdom as a province. The focus of Thai history and politics now moved to the central plains of present-day Thailand, where Ayutthaya was establishing itself as a centralized state, its power outstripping not only Sukhothai but also other neighbouring states such as Suphannaphum and Lawo
Chiang Mai Period
While one of the Thai tribes of the Chao Phraya River was founding Sukhothai kingdom, another tribe in the north-western tableland, called Lanna, was also successful in driving out the Mons influence from the River Ping. In the nineteenth Buddhist century King Meng-Rai of the ancient Chiang-San dynasty was known to have defeated King Ye-Ma, the Mon king of the town of Lamphun, and later built his capital at Chiang Mai.During this time Theravada Buddhism of Ceylon had been brought from their flourishing states in the Mons country and in Sukhothai to the north-western tableland, but was not able to take its firm roots there. In the twentieth Buddhist century through the royal order of King-Kue-Na, several “Lankavangsa” bhikkhus both from Moulmein (Mau-Ta-Ma) and from Sukhothai were invited to Chiang-Mai (750 km. north of Bangkok) to preach their doctrine. Of these bhikkhus along with their followers, one named Ananda was from the town of Mua-Ta-Ma in the Mons country and the other called Sumana was from Sukhothai.In the following century (B.E. 2020 or 1477 AD) under the auspices of King Tilokara, the thirteenth of Chieng Mai dynasty and under the leadership of Khammadinna Thera, a general Council of bhikkhus which lasted one year was convened at the Maha Bodhivong Vihara. Practically this was the first Council held in Thailand and reflected the intensive study of Buddhism during the time. A collection of Pali texts, compiled by the Thera (Elders) of that glorious age, are now a pride of the those who wished to further their research of Buddhism in the Pali language. Some such texts were Abhidhammayojana, Mulakaccayanayojana. Vinayayojana, Vessantaradipani and Mangalathadipani. In the following (twenty-second) century the town was taken by the Burmese and from time Chiang-Mai became a unhappy town alternately torn by two superior powers i.e. Burma on her north and the kingdom of Ayutthaya on her south.
Ayutthaya Period
Towards the close of the nineteenth Buddhist century which witnessed the decline of Sukhothai kingdom, King U-thong of Suphunaphum, once under Sukhothai domination, proclaimed his state as independent of Sukhothai power and built up his capital at a town called Sri Ayutthaya, south of Sukhothai. This kingdom, which lasted 417 years, are ruled over by 33 kings.Through more than four centuries which marked the age of Ayutthaya kingdom, Theravada Buddhism in Thailand seemed to reach its zenith of popularity. Within and without the city of Ayutthaya there scattered innumerable temples and pagodas which served as places, thereby exerting a great influence on the spiritual life of the people. Buddhist art, both in the field of architecture and Buddha-image construction, were on the same line of flourishing. An illustrative example of this fact may be seen today in the temple of the Foot-Prints at Saraburi. There was also a tradition which is still in practice today for every Thai young man to be ordained at least once as a bhikkhu. Several kings such as Pra Borom Trai Lokanatha, the 18th king, in following the example set by King Li-Thai of Sukhothai period, had temporarily renounced his throne to be ordained as a bhikkhu.
During the reign of Phra Borom-Kote, the thirty-first of Ayutthaya kingdom, there reigned in Ceylon a king named Kitti-Siri-Raj-Singha, who being discouraged by the decline of Buddhism in his island country and learning that Buddhism was purer in Thailand than any other country, sent forth his religious mission to the Thai King, asking a favour of some Thai bhikkhus to revive the spirit of Theravada Buddhism which had almost died out in his land. This was a good occasion when Thailand was able to repay her debt to Ceylon and the Venerable Upali, together with his followers, were sent to Ceylon. Thus the community of Ceyl;onese bhikkhus ordained by the Thai bhikkhus at that time has ever since been called Upali-Vangsa or Siam-Vangsa. It is the well known and most revered sect in Ceylon.
Religious literature of Ayutthaya, however, abounded both in Pali and Thai language, but most of them were most regretfully destroyed when the kingdom was ruthlessly overrun by the enemy in 2310 BE.
Thonburi Period
There was not much to say about Buddhism in the short-lived Thonburi period (2310-2365 BE). During the prelude of fifteen years, a greater part of which was occupied in driving our the enemy and restoring the peaceful situation of the country, what could be done to Buddhism was merely a general revival of Buddhism, not to say the compiling of new texts and other measures for the propagation of Buddhism. In the reign of King Thonburi he had several temples repaired, monastic rules settled, religious texts collected and the study and practice of Buddhism revised to some degree. With regard to the texts such as the Tipitaka, Commentaries and Sub-commentaries destroyed by fire, he had them borrowed or copied from those Combodia. It is safe, however, to say that Theravada Buddhism in the form of that of Ayutthaya was still prevailing in Thonburi period. 
