This thesis is about the isnads used by al-Tabari in his renowned Tafslr. It concentrates on the 8,424 isnads used by him in his commentary on the first two suras of the Qur'an, al-Fadha and al-Baqara and seeks to answer the following questions: i) Did he cite his isnads randomly or according to a systematic method? ii) To what extent did he rely on written as opposed to oral sources? iii) Is his claim that he was merely passing on the material of older authorities historically justified? iv) In the light of the theories of Goldziher and Schacht which cast doubt upon the authenticity of isnads, what conclusions can be drawn about the authenticity of those cited by al-Tabari? The thesis is divided into three parts. After an Introduction stating the significance and objectives of this study, Part One (Chapters One to Four) provides background information on al-Tabari's biography, the development of exegesis, and two introductory studies on his Tafslr. Part Two (Chapters Five and Six), which forms the core of the study, presents an analysis of the 8,424 isnads used by al-Tabari in his commentary on Sural al-al-Fatiha and Sural al-Baqara. Chapter Five examines alTabari's style in narrating from his masters and analyses his technical terms of transmission. Chapter Six contains a detailed statistical study of the frequency of alTabari's use of his various isnads. Part Three (Chapters Seven and Eight) reviews the results of Part Two. Chapter Seven discusses the broader implications of these results for the question of the authenticity of al-Tabari's isnads and the possibility of their containing written material from the first and second centuries AH. Chapter Eight examines the theories of Goldziher and Schacht regarding the authenticity of isnads and suggests how one may ascertain whether, and or to what degree, the isnads in alTabari's TafsTr are reliable. Finally, in an Epilogue, we present the most important results obtained by this study.