This Chapter presents an in-depth analysis of the Thai writing system, exploring its historical roots, orthographic conventions, and the linguistic principles that shape Thai script usage. It begins by tracing the development of Thai script from the Old Khmer and Brahmic writing systems, emphasizing its structure as an abugida, where vowels are placed around consonants to indicate sound and tone. The chapter examines the use of specific consonants—particularly ศ, ษ, and ส—in words derived from Sanskrit, Pali, and other languages, illustrating the influence of etymology and phonology on Thai orthography. It also investigates the use of ณ and น, final consonants, vowel forms with or without diacritics (◌ะ), and orthographic phenomena such as duplication, vowel elision, and affixation. Furthermore, it discusses tonal marking, punctuation, and the systematic adaptation of loanwords from Pali, Sanskrit, Khmer, Chinese, Japanese, and European languages. Through a comprehensive linguistic and historical lens, this chapter highlights the intricacies of Thai script and offers readers a foundation for mastering Thai writing with accuracy and cultural awareness.