Vatican I and Vatican II represent two of the three ecumenical councils in modern times, yet relatively few studies have sought to understand their relation to one another. In fact, the councils are often positioned as mutually exclusive so that one must choose either Vatican I's or Vatican II's presentations of church and ecclesial authority. Failing to understand the relationship between these councils inhibits the church's self-understanding and risks misinterpreting key aspects of its own tradition. This book uses the questions of what, why,a nd how the councils taught to frame and demonstrate significant points of continuity, complementarity and difference between them.