[Practice, Problems and Prospects of Ecumenical Processes. A Contribution to Theory Building] Why is ecumenism in need of a theory? The monograph investigates this topical question by seizing on the barely received thesis of the ecumenicist Peter Lengsfeld: Ecumenical processes are determined by an interaction of non-theological and theological factors. Central categories within his theory are truth, social structure and identity of the churches which have a mutual influence on each other in ecumenism. The monograph contributes to the construction of an ecumenical theory by critically reflecting on Lengsfeld's approach. For this purpose the author analyses histories of denominational separations and the practice of ecumenical processes, identifies problems in ecumenism and in theory building and develops prospects for their understanding and possibilities for action.