This work examines the use of mental imagery in private devotion in the Roman Catholic and Eastern Orthodox traditions of prayer. The research is based on the writing of the saints of the two Churches, as well as on the analysis provided by some of the best theologians of the Russian Orthodox Church. The findings are to be used as a tool within the ecumenical dialogue between the two Churches. The core of the argument is that the two traditions followed significantly different paths in their approaches to spiritual life. These differences exist in many aspects of devotion, but can be exemplified by the favorable view of the use of imagination in Roman Catholic prayer and the caution with which it is approached in the Eastern Orthodox Church. The difference in devotional paths and the standards of prayer that have been canonized through the glorification of saints by each Church may present a much bigger challenge to the task of reconciliation between the two Churches than heretofore has been acknowledged. This work highlights the reality and significance of the differences between the two traditions and urges the continuation of the research in order to advance the dialogue between the Eastern Orthodox and Roman Catholic Churches.