The dissertation is devoted to the study of the vowel system of an Arabic variety spoken in the major urban centers in northern Egypt, known under the generic term “Egyptian Arabic.” The first common thread that runs through all chapters of the dissertation is to establish descriptive generalizations on issues that have not yet received the proper attention, descriptions that constitute true linguistic generalizations of a type likely to feed theoretical elaboration. New relevant information will be cited and a detailed analysis of a comprehensive fragment of fresh data using analytical tools different from those previously used by my predecessors will be offered. The main argument of the dissertation is that the key to the interpretation of the most prominent vowel-related events in this language lies in the interdependence of vowel quality and vowel quantity. I establish a transparent relationship between these two, only phonologically long vowels in Egyptian Arabic can maintain their quality on the phonetic level, otherwise they become incapable of surfacing. They are, in this case, phonetically replaced either by an epenthetic vowel or by nothing at all. I thus construe vowel length in strictly phonological terms. The establishment of the new construal of vowel length in Egyptian Arabic was also made possible through comparison with other varieties of Arabic whose diachrony and parameters are relatively well understood, namely Classical Arabic and the Arabic spoken in the Maghreb. With the proliferation of valuable research on contemporary varieties of spoken Arabic, the prospect of defining the parameters responsible for the identification and stabilization of each language or dialect, appears to be within reach. This is the other common thread running through the dissertation. The dissertation in this respect contributes to the enhancement of the study of Arabic dialectology. But, my ultimate goal is to give a deductive structure to my results and to draw conclusions that can go beyond the case of Egyptian Arabic or Arabic dialectology, indeed to contribute to the elaboration of Linguistic Theory.