It has been asserted that monotheism, in the Jewish tradition, has long been understood both exoterically and esoterically. In the exoteric Scripture-based rabbinic tradition, monotheism is the belief in a one and only God, a belief which goes hand in hand with the affirmation of distinct individual and divine existences, so that there is a dualism between humanity and God. In the esoteric or mystic tradition, this dualism is overcome by a conception of monotheism in which God is One, not only in his 'Lordship' but also in his universal reality. That is, God is the only reality, so that everything which exists is in essence an aspect of divinity. Jewish mysticism has both a devotional or practical and an intellectual or speculative side. On its devotional side it emphasizes those aspects of the biblical precepts which serve to promote direct communion between the worshipper and God. On its speculative side it is especially concerned with outlining and bringing into relief the link or links between God and man, or more generally between the Creator and the universe. The focus of this study is on the questions of how and why Jewish mysticism arose and underwent a variegated evolution throughout much of the history of the Jewish people from remote antiquity to the present day.