Discussion of theosis is mainly undertaken within Byzantine Orthodoxy and is usually structured around an examination of patristic and medieval sources. This work builds upon such discussions and broadens the basis by including Western sources (Catholic and Protestant) in contemporary debates about salvation. In seeking to explore a relational understanding of salvation, the construal of the doctrine of deification is examined in terms of the divine nature understood as communion. This is related in particular of the theological project of Zizioulas, and other exponents of communion ontology. The book is an investigation of how understandings of theosis in the Christian Tradition have related to understandings of divine nature in terms of koinonia. It further suggests a relational paradigm for conceptualising how theosis may be understood today, drawing out implications for the Christian community and discipleship.