This chapter considers two important ecumenical councils of the Church in the modern era: Trent (1545–63) and Vatican I (1869–70). The chapter examines in detail the key teachings of each council. The reform decrees of Trent will only be discussed in so far as they touch upon dogmatic decrees. In the case of Trent the chapter identifies the key documents from the many sessions of the council over its twenty-year history, offering a clear guide to ways in which its teachings on revelation, grace, and justification offered a precise Catholic response to the emergent theologies of Protestantism. Vatican I’s key teachings on revelation, the knowledge of God, and the status of the papacy are similarly treated.