The Thai language is an important part of Thai national identity. Even though Thailand consists of various ethnic groups with their own languages, Thai has been regarded as the lingual franca and has been used for communication among people for at least 700 years. The importance of Thai is attested by Sukhothai and Ayutthaya inscriptions found in many regions of the country. It is also made evident by a number of ancient literary works which have been preserved until our time. However, we still have little knowledge of the historical development of Thai. This is a major obstacle for in-depth research of Thai epigraphy, literature and pre-history. Therefore, study of the Thai language from historical linguistic perspectives can bring about a better understanding of Thai society, politics and culture in the past.