This book explores the origins and development of one of the most significant doctrines of Reformation theology. The innovative ways in which the Zurich reformer Huldrych Zwingli and his successor Heinrich Bullinger thought about the relationship between the Old and New Testaments left an indelible mark on the Reformed tradition in the sixteenth and seventeenth centuries. Distinctively, Zwingli and Bullinger emphasized the continuity of both testaments and spoke of a single covenant between God and humanity. This would become one of the defining teachings of Reformed Christianity. This book follows the development of their "covenant theology" in the Reformation and argues for its adoption by John Calvin in Geneva and the German theologians of the post-Reformation era.