The idea of "holiness" is central to religion, but it is also one of the hardest concepts to define. In Judaism, the word kodesh, the Hebrew word for "holy," appears hundreds of times throughout Tanakh and the Talmud. It also appears in ancient Near Eastern contexts, throughout rabbinic literature, and is used throughout the Middle Ages and into modern times. Is "holiness" a synonym for Godliness, one of God’s attributes, or does it have independent existence? What does it mean to say that both God and man are holy? What is the proper understanding of "Be holy, because I the Lord your God am holy"? A Theology of Holiness analyzes the meaning of the Hebrew root k-d-sh from ancient sources, throughout Tanakh, the Talmud, Rashi, Maimonides, Nahmanides, Rabbi Samson Raphael Hirsch, and Rabbi Joseph B. Soloveitchik. This work traces how the idea of holiness has been applied throughout the ages. It is a work that utilizes historical, exegetical, linguistic, literary, anthropological, and philosophical tools in an interdisciplinary analysis. Ultimately, it is a work of theology, and helps the reader achieve a deeper understanding of holiness—arguably the most important religious term of them all­.