Proto-Tai is the ancestor of the Tai languages of Mainland Southeast Asia. Modern Tai languages share many structural similarities and phonological innovations, but reconstructing the phonology requires a thorough understanding of the convergent trends of the Southeast Asian linguistic area, as well as a theoretical foundation in order to distinguish inherited traits from universal tendencies, chance, diffusion, or parallel development. This dissertation presents a new reconstruction of Proto-Tai phonology, based on a systematic application of the Comparative Method and an appreciation of the force of contact. It also incorporates a large amount of dialect data that have become available only recently. In contrast to the generally accepted assumption that Proto-Tai was monosyllabic, this thesis claims that Proto-Tai was a sesquisyllabic language that allowed both sesquisyllabic and monosyllabic prosodic words. In the proposed reconstruction, it is argued that Proto-Tai had three contrastive phonation types and six places of articulation. It had plain voiceless, implosive, and voiced stops, but lacked the aspirated stop series (central to previous reconstructions). As for place of articulation, Proto-Tai had a distinctive uvular series, in addition to the labial, alveolar, palatal, velar, and glottal series typically reconstructed. In the onset, these consonants can combine to form tautosyllabic clusters or sequisyllabic structures. Regarding the rime, PT had seven vowel qualities that contrasted in height, backness, and rounding. A vowel length contrast also existed for each quality. Palatal and lateral consonants also occurred in the coda in addition to the final consonants generally assumed. Furthermore, Proto-Tai was a tonal language whose four tonal categories *ABCD contrasted both in terms of pitch and voice quality. Many of these Proto-Tai traits are not attested in modern Tai languages. The current reconstruction of Proto-Tai phonology is thus a demonstration of the power of the Comparative Method as well as the role that phonological theory can play in reconstruction. This thesis presents a picture of the history of Tai languages as characterized by divergent changes overridden by waves of convergent trends that transformed Proto-Tai into a network of typologically homogenous dialects that differ markedly from their parent. This analysis offers a comprehensive account of the transformation of the Proto-Tai phonology into modern systems.