One of the most important questions in Australian comparative linguistics over the last 40 years is the validity of a Pama-Nyungan node in the Australian family tree. Much of the comparative research done on Australian languages has supported the notion of a Pama-Nyungan family, and its validity is now well-established. However. much work remains to be done, both in establishing the relationships among the Pama-Nyungan languages and in reconstructing proto-Pama-Nyungan and determining the details of its development in the various branches of the family tree. This dissertation is a contribution towards the latter effort. The primary purpose of the present study is to determine the development of the three initial velars, *k. *ng and *w. in 25 Pama-Nyungan languages through 1561 cognate sets. The cognate sets are also an important resource for the study of other aspects of phonological change in Pama-Nyungan languages. The data provide evidence for the weakening of medial consonants, the assimilation of initial velar glides and nasals to the following vowel, prenasalization of medial stops, the development of triconsonantal clusters, and the presence of both a laminal lateral and a retroflex series of consonants in proto-Pama-Nyungan. In addition, statistical evidence is presented which supports the hypothesis that assimilation of the second to the first vowel is an important process in the history of many Pama-Nyungan languages. This dissertation also discusses important issues regarding the Neogrammarian hypothesis and the comparative method. In particular, the data presented here support the idea that not all sound changes apply in a lexically abrupt, regular manner. Many of the sound changes seen in the data appear to affect only a portion of the eligible forms, and thus provide evidence for the theory of lexical diffusion. Furthermore, most of the changes are found not just in individual languages, but in a number of the languages under study. The data therefore support the notion of pandemic irregularity. / Graduate