Unto the Churches of Christ Everywhere (1920). This Encyclical letter of the Greek Orthodox Patriarchate of Constantinople is considered to be foundational for the establishment of the World Council of Churches.
Decree on Ecumenism (Unitatis Redintegratio, 1965). This document from the Second Vatican Council is the starting place for understanding the Catholic approach to ecumenical dialogue among separated Christian churches. Sections 14-18 deal with relations between the Roman Catholic and Eastern Churches.
Directory for the Application of Principles and Norms on Ecumenism (Rev. Ed., 1993) First published in 1967-70, for the purpose of promoting an ecumenical spirit in the Catholic Church, along the lines established by Unitatis Redintegratio: "The new edition of the Directory is meant to be an instrument at the service of the whole Church and especially of those who are directly engaged in ecumenical activity in the Catholic Church. The Directory intends to motivate, enlighten and guide this activity, and in some particular cases also to give binding directives in accordance with the proper competence of the Pontifical Council for Promoting Christian Unity." (§6)
On Commitment to Ecumenism (Ut unum sint, 1995). Encyclical letter of Pope John Paul II.
Basic Principles of the Russian Orthodox Church's Attitude to the non-Orthodox. The official position of the Moscow Patriarchate.
Joint International Commission for Theological Dialogue Between the Catholic Church and the [Eastern] Orthodox Church
Joint Declarations and Major Addresses:
Ecumenical Patriarchate of Constantinople
Orthodox Patriarchate of Romania
Orthodox Patriarchate of Bulgaria
Orthodox Church of Greece
North American Orthodox-Catholic Theological Consultation. Documents from various levels of Catholic-Orthodox Ecumenical Dialogue may be found at the website of the United States Conference of Catholic Bishops (USCCB), Secretariat for Ecumenical and Interreligious Affairs (SEIA): www.usccb.org/seia/