Rather than a "mysticism of everyday life," Karl Rahner advances an understanding of mysticism rooted in the ongoing historical interactions of human freedom with the life of the God of Jesus Christ. This dissertation reconsiders Rahner's approach from three perspectives. In Chapter One, I position Rahner within Catholic debates regarding mysticism at the beginning of the twentieth century. Anti-rationalist and psychological uses framed mysticism as a unique knowledge generated by extraordinary phenomena. Building upon neo-Scholastic conceptions of faith, reason, and intuition, theologians debated about the universal call to mysticism and the essence of "infused contemplation." These debates form an essential background for Rahner, who extends the approach of Auguste Saudreau and Maurice de la Taille while firmly opposing that of Augustin Poulain and Joseph Maréchal.In Chapter Two, I survey Rahner's treatments of mysticism throughout his career. Though he emphasizes non-beatifying immediacy to God in the 1930s, Rahner undergoes a major shift when he begins treating mysticism in terms of "mediated immediacy." From 1944-1970, he connects and distinguishes features of the relationship between extraordinary mystical and ordinary faith experiences. From 1970-1984, Rahner describes mysticism as the theologically significant union of a human's radically free openness to God and concrete enactments of this acceptance of grace.Third, I highlight affinities between Rahner and three mystical theologians. In Chapter Three, I compare Rahner's approach to mysticism with his interpretation of Ignatius Loyola's Spiritual Exercises, which reveals the often overlooked ecclesial and apostolic aspects of his understanding of mysticism. In Chapter Four, I highlight similarities between Rahner's account of loving knowledge before the incomprehensible God and Bonaventure's understanding of the ecstatic shape and goal of human knowing. I also examine how Jan van Ruusbroec can illuminate the Trinitarian features of Rahner's understanding of mysticism.  In Chapter Five, I offer a synthetic account of Rahner's approach to mysticism and situate it in relation to his theological system, with special attention to the relation of human freedom and knowledge, salvation and revelation history, as well as Christology and ecclesiology.