Mystics who have spoken of their union with God have come under suspicion in all three major religious traditions, sometimes to the point of condemnation and execution in the case of Christianity and Islam. Nevertheless, in all three religions the tradition of unio mystica is deep and long. Many of the spiritual giants of these three faiths have seen the attainment of mystical union as the heart of their beliefs and practices. Despite its importance, mystical union has rarely been investigated in itself, apart from the wider study of mysticism, and even more rarely from the aspect of comparative studies, especially those based upon broad and expert knowledge of the inner life of the three related monotheistic faiths. This text brings together essays that equally explore the broader idea of unio mystica as well as the mystic traditions within each religion.