Book excerpt: Gaudium et Spes, Vatican II’s Pastoral Constitution on the Church in the Modern World, marked fundamental shifts in ethical methodology, in how we do ethics in the Catholic tradition, and in how we think about ethical and ecclesial issues in the Catholic Church in the modern world. On the document’s fiftieth anniversary, this book explores the historical origins of Gaudium et Spes, its impact on the Church’s ecclesial self-understanding, and its implications for doing Catholic theological ethics for the specific ethical issues of marriage, social justice, politics, and peacebuilding.The book engages in the ongoing communal discernment of the aggiornamento sought by the council’s convener, Pope John XXIII, seeking to bring the Church up to date in the twenty-first century.