This Chapter provides an extensive introduction to the Thai language and writing system, emphasizing the structural, phonological, and historical components that define Thai orthography. It begins by outlining the unique characteristics of the Thai script as an abugida, where consonants inherently carry vowel sounds, and diacritics indicate tones and other phonetic features. The chapter details the current inventory of 42 consonant graphemes representing 21 phonemes, including classifications into mid-, high-, and low-class consonants, and discusses their roles in tonal determination. It also introduces consonant clusters, silent consonants, and loanword adaptations, highlighting how Thai phonology accommodates both native and borrowed lexical items. Additionally, the chapter examines the structure and pronunciation of Thai vowels—both monophthongs and diphthongs—and their various transformations, omissions, and classifications. The discussion concludes with an overview of Thai tones and syllable structures, and a brief historical account of the script’s development within the Tai-Kadai language family. This foundational chapter equips readers with essential knowledge for understanding Thai phonology, orthography, and linguistic heritage.