Questions surrounding the understanding of "communion" are a significant feature of much contemporary ecclesiology, but their 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 through an investigation of the concept of communion in the work of Jean-Marie Tillard, OP. Tillard's theology is noted 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, thereby framing an answer to the contemporary question of ecclesial unity and diversity. The book also assesses 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.