For centuries prior to the Reformation, Christian theologians identified Adam’s natural relationship to humankind as the basis for its participation in his own guilt and corruption. Beginning in 1532, the Dominican theologian Ambrogio Catarino challenged the validity of this traditional dogma, arguing that humankind’s solidarity with Adam stemmed immediately from divine volition and ordination. According to Catarino, Adam’s appointment to bear the moral cause of every human person was concretely embodied in a covenant that God established with Adam in the garden. Catarino’s teaching sparked several decades of moderate controversy among Roman Catholic thinkers regarding the reality of a pre-fall covenant and the proper basis of humankind’s solidarity with Adam. In the long run, his teaching found few Catholic supporters. However, from the late sixteenth-century onwards, Protestant Reformed theologians began to advance a notion of humankind’s solidarity with Adam remarkably like Catarino’s doctrine. For Reformed theologians, it was the so-called ‘covenant of works’ that provided a rationale for universal Adamic guilt. That covenant performed other conceptual tasks in the mature theological systems of post-Reformation Reformed thinkers, but grounding humankind’s solidarity with its common forefather was not least among the operations ascribed to it. This book exposits Catarino’s own doctrine of covenantal solidarity, explores the medieval sources of his teaching, and traces the influence of his doctrine on Reformed thinkers. It provides insight into the doctrine of a significant Catholic Reformation theologian, and illumines the complicated and somewhat surprising background to Reformed theology’s own eventual teaching on Adam’s federal (covenantal) headship.