Theosis, originally a Greek term for Christian divinisation or deification, has become a vogue word in modern theology. Although recent publications have explored its meaning in a selection of different contexts, this is the first book to offer a coherent narrative of how the concept of theosis developed in both its Eastern and Western versions. Norman Russell shows how the role of Dionysius the Areopagite was pivotal, not only in Byzantium but also in the late mediaeval West, where it strengthened the turn towards an individualistic interiority. Russell also relates theosis to changing concepts of religion in the modern age. He investigates the Russian version of theosis, introduced in the West by Russian members the Paris School after the 1917 Revolution. Since then, theosis has undergone additional development through the addition of esoteric elements which have since passed into the mainstream of all theological traditions and even into popular spirituality.