Researching Ecumenism
World Council of Churches Document Center
Free, downloadable copies of many of the WCC's policies, addresses, and meeting minutes. A wealth of primary source material!

National Council of Churches
NCC's website is not particularly useful. Policy statements can be found on the "Commons Witness" page. If you are interested in visiting the official archives of the NCC, you can do so in Philadelphia, as it's hosted by the National Archives of the Presbyterian Church (USA). Please check their website for information on using the archives.

Global Digital Library on Theology and Ecumenism (requires free registration)
GlobeTheoLib is a vast resource on ecumenism and ethics, including over 2,000,000 full-text items. This digital library is created and maintained by the World Council of Churches. It contains both scholarly material as well as material uploaded by researchers, as well as news items, etc.

US Conference of Catholic Bishops
Ecumenical Documents and News Releases from the Catholic Church in the US.

"Martin Luther in the Ecumenical Movement," by Sarah Hinlicky Wilson, in Oxford Research Encyclopedia of Religion. 2016.
Phrases for searching
"Relation with other churches"
"Church Unity"
"and other churches"
Person and Ecumenism
     e.g.: "Zwingli and Ecumenism"
"Christian Unity Movements"
"Ecumenical movement"
"WCC" or "NCC"
World Council of Churches
National Council of Churches
American Board of Commissioners for Foreign Missions
American Home Missionary Society
Call Nos. on Ecumenism
BV 186 
     Ecumenical Worship
BX 1 to BX 9.5
     Global Ecumenism (WCC, etc.)
BX 1779 to BX 1788
      Catholic Church and ecumenism
BX5925 to 5928
     Episcopal church and ecumenism
BX8063 
     Lutherans and ecumenism
BX8329
     Methodists and ecumenism
BX9416
     Puritans and other churches
BX9419
      "Reformed and Calvinist" and other churches