This dissertation discusses the phonetic interpretation of the Tibetan alphabet, in particular certain controversial letters 'h[dotbelow]', 'w', and 'y'. Chapter 1 suggests that aspiration is not a phonemic contrast in Old Tibetan, and that a theory of the phonetic word explains the distribution of unaspirated consonants. A series of three studies then treats the letter 'h[dotbelow]'. Chapter 2 provides an overview of relevant evidence for the pronunciation of this character in all syllable positions. Chapter 3 looks in detail at Anlaut position and phonemically analyzes the phonetic data already arrived at. Chapter 4 addresses the cognate character [Special character omitted.] in the H[dotbelow]phags-pa script, attempting to reconcile my interpretation of the use of its Tibetan relative with the evidence from Mongolian and Chinese. Chapter 5 addresses a controversy regarding the letter 'w', which I argue was pronounced as /w/ in Old Tibetan. An understanding of the phonetic values of both 'h[dotbelow]' and 'w' facilitates the examination in Chapter 6 of the interchange between 'b' and 'h[dotbelow]' found both in Old Tibetan texts and in the modern Tibetan languages. Chapter 7 considers how the two ways of writing a 'g' followed by a 'y', transliterated as