Although it is the world’s second largest Christian denomination, with between 250 and 300 million adherents, Orthodoxy remains a little known and “mysterious” entity in the West. Yet for Orthodoxy, it is simply a way of being Christian ultimately rooted in the person of Jesus and the experience of the early church. The Introduction explains how Orthodoxy and Catholicism split between the eleventh and thirteenth centuries, and then developed along separate trajectories. Among the difficulties for modern Orthodoxy is the lack of any universal head, which prevents it from presenting itself to the world as a united entity. Despite this, Orthodox Christianity continues to thrive, both in its traditional homelands and in the West.