This book is a Scientific Study on one of the endangered languages in Ethiopia. It describes the phonology of Zay language which is spoken by people living in Zway Island. The book consists of five units. Unit one provides a brief description of the people, the language and the theoretical framework employed. Unit two gives the overview of consonants and vowels in the language. The distinctive features that are relevant to account for the phonemes of the language are identified and discussed in terms of underspecification theory in here. Unit three deals with the syllable structure and their constraints. The morphophonemic processes and the relationship they have with the syllable structure is discussed under unit four.Unit five comprises of the summary and conclusion of the book. Is is believed that the book is very important for those people who are interested in the scientific study of language f0r it is based on one of the modern theories of language studies on phonology.