What does the concept of 'communion' mean for present day understanding of the Church and Ecumenism? The use of the term 'communion' is a significant component of much contemporary ecclesiology, but its prominence calls attention to wider questions regarding ecclesiological method. Brian Flanagan addresses the questions of how to characterize a systematic ecclesiology and the possibility of a systematic communion ecclesiology by investigating the concept of communion in the work of Jean-Marie Tillard, OP. Tillard's theology is notable as the most prominent Roman catholic communion ecclesiology. Flanagan argues, that Tillard contributes to systematic ecclesiology by defining the concept of communion in relation to Christology, soteriology, and theological anthropology as an answer to the contemporary question of ecclesial unity and diversity. It also analyses the danger of idealism in Tillard's thought and suggests that further engagement with social scientific study of the church will help strengthen, nuance, and critique Tillard's idea of communion. "Ecclesiological Investigations" brings together quality research and inspiring debates in ecclesiology worldwide from a network of international scholars, research centres and projects in the field.