This text introduces the reader to the works of the great Muslim scholars and jurists who laid the foundations of Islamic jurisprudence. It reviews the history, origins and main characteristics of the four Sunni and Shi'ite Imami schools of jurisprudence. The urgent need for ijtihad required that principles (usul) be laid down as to how to deduce and derive laws from the main sources (masadir), namely, the Qur'an and the Sunna. The new circumstances and the emergence of different creeds and sects, and of course the expansion of Islam, necessitated the development and articulation of such principles, which could direct and guide the process of ijtihad within the framework of Shari'a and its general purposes. Thus, the reader will not only be introduced to Islamic juridical vocabulary and terminology, but will also grasp and discern the evolutionary and historical contexts within which schools of jurisprudence were developed.