The records of the Third Ecumenical Council of Ephesus, which was held in 431, form the most important source basis for studies on the first phase of the Christological disputes of the 5th century. They were edited by Eduard Schwartz between 1923 and 1929 as the first major volume in the series Acta Conciliorum Oecumenicorum. This collection includes not only documents that illustrate the proceedings and negotiations of the actual council but also presents, through its selection of texts, the entire history of the dispute that arose over the question of whether it was compatible with the right faith to refer to Mary, the mother of Jesus, as the "Mother of God": from its beginnings in 428, through the council convened to clarify the issue, to the agreement reached in 433 and the immediate aftermath of the conflicts. The volume presented here is the first in a series aimed at fully translating the documents of Schwartz's collection into a modern language for the first time, providing detailed commentary and arranging them in an order that makes the proceedings of the council and its history immediately transparent to the reader.