Aion is one of the major works of C.G. Jung’s later years. The title comes from the Greek word for aeon or age and refers to the age of Christianity, for, in Aion, Jung is concerned with the collective psychic development that the Christian era represents. How did it come about when it did? What psychic change did it represent? In exploring these questions, Jung (1875-1961) draws upon Christian symbolism and, in particular, the figure of Christ as a case study in the archetype of the Self. The book begins with an account of the important concepts of the ego, the shadow, and the anima/animus (here referred to as the syzygy). Jung then continues with his exploration of the Self archetype as represented through Christian symbolism and the figure of Christ. In particular, a discussion and critique of the Christian view of evil is provided. The symbolism of the fish, which recurs as a symbol of both Christ and of the devil, is examined in detail and a wide range of alchemical, pagan and gnostic material is drawn upon. The final chapter of the book proposes a model for the structure of the Self. Aion is one of the most important volumes of the Collected Works and complements The Archetypes and the Collective Unconscious.