We are working on a theology of the sister churches that can become governing for the koinonia of the Catholic and Orthodox local churches. We take up the impulse of Pope John Paul II, who in his encyclical "Ut unum sint" says: "The traditional term "Sister Churches" should constantly accompany us on this path[of the restoration of the unity of the Church]" (UUS 56). We owe a historical-theological review of the topic to the Orthodox theologian Prof. Will Cohen, whose dissertation is published in the "Studia Oecumenica Friburgensia": "The Concept of 'Sister Churches' In Catholic-Orthodox Relations since Vatican II" (= SOF 67, Münster 2016).