The focus of this work is the first definitional theory of truth in the history of philosophy presented by Anselm of Canterbury. It presents a detailed philosophical reconstruction of Anselm’s theory in the context of his whole thinking and with a special consideration of his antique sources in Aristotle, Cicero, Augustine and Boethius. After an analysis of the different meanings of truth in Monologion and Proslogion, Anselm’s dialogue De veritate and the continuity of its understanding of truth in Anselm’s later works is investigated. Topics covered include the purpose and the topical method of De veritate and its source in the boethian topics, the coherence between Anselm’s theory of truth, of modalities and his soteriology, and a comprehensive study of the preanselmian history of the ideas of conditional and simple necessity in Aristotle, Augustine and Boethius.