A clearly significant objective of political Islam is to assist Muslims to establish a lawful government or caliphate which is in turn capable of safeguarding the interests of the Ummah and the purity of religion as well. Since religion is a most sensitive issue to the Muslim, and more especially to the `Ulama, we find that many of the disturbances and great political unrest occurring in the world of Islam have been, to a large extent, due to the failure of the ruling government to attain this objective. The achievement of the `Abbasid party in the overthrow of the Umayyads in the early decades of the second century Hijri (eight century Masihi) was very much related to such a failure. Thus, this change not only substituted one dynasty for another, but further had substantial and far-reaching political, religious and social consequences. Inasmuch as this study is deeply concerned with the fundamental task of the `Ulama, I have therefore attempted to view and inspect all circumstances in accordance with the orthodox standpoint. Hence, in chapter one of this study, I endeavour to examine the causes which brought the `Abbasids to power, as well as the reasons for the fall of the Umayyad caliphate. Since it is apparent that this change could hardly have succeeded without the cooperation and support of the `Ulama who controlled the masses at that time, in chapter two, a full discussion of the nature, purpose and institution of the `Ulama in the contemporareous Muslim community is provided. This reveals the true features and characteristics of the `Ulama in a real religious sense rather than in the ordinary meaning of `learned men' or `scholars'. Yet, assuming the roles and activities of the `Ulama were extremely wide (while, of necessity, space here is limited), discussion has accordingly been focussed in the following chapters on scrutinizing the central role of the `Ulama in the field of jurisprudence. Thus, in chapter three, the historical development of the different schools of law, as well as the various methodologies introduced and employed by each school, are examined. The effect of these developments is treated in chapter four, where we consider the growth of the four orthodox <i>madhhabs</i> and the points of difference between these schools of law. Chapter five deals exclusively with the development of <i>ijtihad</i>, the key factor which keeps the legal system of Islam functioning, flexible and acceptable. The rise of various problems in the community forced the `Ulama to adopt new solutions, and this made the process of <i>ijtihad</i> alive and variable. But, the glory of this development was disturbed when Caliph al-Ma'mun moved from orthodoxy to rationalism, and started to persecute those who opposed him. The result of this <i>mihnah</i> (inquisition) raised enmity and controversy between the Traditionalists and the Rationalists which consequently effected the development of ijtihad. This is carefully discussed in chapter six in which we also try to review the idea of <i>taql{i}d</i> that seems to have gained ground in the Muslim community at that time. Since this concept was entirely traditionalist, it is therefore hard to accept it as a legitimate rule of <i>shar{i}'a</i>. Thus, in summing up this study, it is concluded that <i>ijtihad</i> was not only important in keeping the law of <i>shar{i}'a</i> up to date, but also served as an effective channel through which the `Ulama could exercise their mental capacities as well as contributing their services to the Ummah.