The present thesis aims to give a phonetic and phonological account of how the speech of a Thai baby, aged 3-18 months, emerges from early vocalizations. The phonological description is made within the framework of 'prosodic analysis'. The introduction includes information on related literature, a brief outline of Thai phonology, the method of collecting data, the theoretical basis of prosodic phonology, the background of the child and adults concerned in the study, and a glossary of notations and terms used in the thesis. The phonetic and phonological development is presented in Section I where the 10 stages of speech development are described with reference to phonetic development, vocal play, relationship between adult models and baby's responses, and phonological development. Adult-baby interaction and adult language addressed to the baby are described in Appendix 1. The findings of the study are summarized in the conclusion and discussion section where a general picture of the speech development and the acquisition of features of the Thai language; tones, aspiration, and final glottalization, are discussed. Examples of adult-baby 'dialogues' as well as of the baby's vocalizations and vocabulary are given in Appendix 2.