Avraham Yitzhaq Ha-Cohen Kook (1865-1935) stands as a colossal figure of modern Jewish history and thought. Jurist, mystic, poet, theologian, communal leader, founder of the modern Chief Rabbinate and still the defining thinker of Religious Zionism, he is indispensable for understanding modern Jewish thought, the contemporary State of Israel, and the most fundamental interactions of religion, nationalism, ethics and spirituality. Despite countless studies of him, almost no full-fledged intellectual biography of him exists in any language. This study of the years before his momentous move to Jaffa in 1904, drawing on little-known works, including recently published manuscripts, begins to fill that gap. It traces his life and times in the remarkably intense Rabbinic intellectual milieu of late nineteenth-century Eastern Europe, and his path from a profound, regularly rationalist traditionalism, towards a dynamic theology and spiritual practice weaving together Kabbalah, philosophy, universal ethics, and romantic mysticism.