This study is an analysis of the political behavior of the clergy in Iran during the Pahlavi monarchy (1925-1979) within the context of the interrelationship between religion and modernization. This period itself has been divided into three eras: the Riza Shah era (1925-1941), the nationalist era (1941-1953), and the post-nationalist era (1953-1979). The analytical framework adopted here is based on three behavioral motives for clerical political involvement: religious, national, and clerical corporate interests. Within this framework, various patterns of clerical political behavior are identified, each reflecting a different order of priority of interests and motives for political action. The main point emerging from this approach is that at no time throughout this period did the clergy act as a politically monolithic group. Such distinct patterns are particularly discernable during the nationalist era (1941-1953) because of the presence of greater socio-political freedom relative to the other two major eras. A major theme developed in this study is that while the greater functional differentiation and specialization caused by modernization resulted in the clergy's loss of influence and official power in the educational and legal systems, the popular base of their political power vis-a-vis the state increased because of the clerical institution's disengagement from the government. Furthermore, the emergence of a new religious intellectual class in the process of modernization led to the ideologization of Islam which in turn was a major causal factor behind the Islamic revolution and legitimization of clerical leadership. Concurrent with this development, the core of the clerical institution underwent a process of politicization through the rise of Ayatullah Khomayni in the 1960's. The Islamic revolution of 1979 represented the mutual reinforcement of these two developments and their convergence in the direction of the establishment of an Islamic system of government.