This book explores the history of the theme of 'union with Christ' in the Reformed tradition. After chapters on the legacy of Calvin and Reformed Orthodoxy, the author uncovers three trajectories in American Reformed theology in which salvation as union with Christ is understood in remarkably different ways. The subsequent twentieth-century history of the theme is also explored. This detailed examination of New England Calvinism, Princeton Calvinism, and the Mercersburg Theology highlights the historic diversity present in Reformed thought, and the implications of that diversity for contemporary Evangelical and Reformed thought.