Jan Van Ruusbroec (1293-1381), a Flemish mystical theologian, was one of the most original Trinitarian thinkers in the medieval West. Yet, his works - written in Middle-Dutch - have remained relatively unknown. In this book, Rik Van Nieuwenhove presents the first major study in English of Ruusbroec's thought. Van Nieuwenhove explores in detail Ruusbroec's theology of the Trinity, his anthropology, Christology, and his understanding of union with God. Van Nieuwenhove's study reveals that Ruusbroec, while incorporating aspects of the rich theological and spiritual tradition that preceded him, wrote at the beginning of a modern age in which mystical theology changed radically in nature. Ruusbroec claimed that the divine Persons are subject to an eternal dynamic of procession or out-going from the Father, on the one hand, and returning to the shared divine essence, on the other. The human person is called to participate in this continuous ebbing and flowing by leading a life that combines contemplation and charitable activity. Ruusbroec argued that mysticism should be interpreted in terms of a transformation of the human person rather than in terms of an immediate experience of God.