This thesis treats the Arabic dad sound in detail from the Pre-Islamic period to the present day. It consists of five chapters; the first chapter, which can be considered as an introduction, treats this sound in the Semitic Languages and in the Proto-Arabic Dialects. The second deals with the historical materials concerning this sound as found in the Arabic sources. The third treats the definition of the articulation of dad as found in the Arabic scholars, beginning from Sibawayhi until the present day. The fourth is a study of the literature on the dad and za' written by Arab scholars between the fourth century A.H. and the fourteenth century A.H. The last chapter presents three edited texts on dad and and za'.