This thesis deals mainly with the historical development of the religious institution of Ithna ashari Shi'ism in both its scholarly and political aspects. It is divided into six chapters. The word "school" has been used to describe the place in which such an institution had flourished due to the activities of its fuqaha ' in response to their turbulent history, whether it was in Iraq, in Bilad al-Sham (Greater Syria, i.e. Syria and Lebanon) or in Iran. Chapter one deals with the Baghdad School. It includes a study of the scholarly development right from the begining of the fuqaha' institution during Shaykh al-Mufid's times (d. 413/1022) and ending with Shaykh al-Tusi (d. 460/1068). Chapter two follows the development of this scholarly renaissance at the hands of the Hilla fuqaha starting with Ibn Idris al-Hilli's time (d. 598/1201) and ending with Fakhr al-Muhaqiqqin ibn al-'Allama al-Hilli (d. 771/1369), and investigates the relationship between the religious institution and the Mongol invaders of Iraq and the ideological influence of the Ithna'ashari fuqaha' on the leaders of the invaders. Chapter three, on the Jabal 'Amil school, deals in part with the unsettled period of the Mamluk state, its struggle against the Mongols and the internal situation of the Shi'a vis-a-vis the Mamluks. It also deals in part with the influence of the Jabal 'Amil fuqaha' on the Safawid state after these fuqaha' had migrated there. Particular attention is paid to the role of Shaykh al-Karaki (d. 940/1533) and his attempt to build a religious institution inside Safawid Iran, and the opposition that he met. The chapter ends with a study of the Akhbari Movement in its first stage, during the time of Muhammad Amin al-Astarabadi (d. 1033/1624). Chapter four focuses on the Najaf School, which had started about two hundred years before as an intellectual school. The development and activities of this school from the beginning of the thirteenth/nineteenth century, are discussed, as is its position regarding the emergence of the Wahhabi Movement, the Akhbari Movement (in its second phase) and the Shaykhi Movement. The chapter also deals with the political activity of the fuqaha' in their struggle against the Qajari state, which had been manifested in the fatwa prohibiting tobacco and in the Constitutional Movement. Chapter five deals with the struggle of the Najaf fuqaha' from the start of the Republican period (1958) until the beginning of the 1990s. This is preceded by an introductory remark concerning the position taken by the fuqaha' towards the British forces who entered Iraq after the First World War and the events of the Iraqi Revolution of 1920. Chapter six has been dedicated to a study of the Qumm school. It looks at the historical development of that city, with particular attention to the role of Shaykh 'Abd al-Karim al-Ha'iri al-Yazdi (d. 1355/1936) in supervising an elite of mujtahids who have participated in the renewal of this city.