The Halki seminary, formally the Theological School of Halki, was founded on 1 October 1844 on the island of Halki, the second-largest of the Princes' Islands in the Sea of Marmara. It was the main school of theology of the Eastern Orthodox Church's Ecumenical Patriarchate of Constantinople until the Turkish parliament enacted a law banning private higher education institutions in 1971. The theological school is located at the top of the island's Hill of Hope, on the site of the Byzantine-era Monastery of the Holy Trinity. The premises of the school continue to be maintained by the monastery and are used to host conferences. It is possible to visit the island where it is located via boat in approximately one hour from the shore of Istanbul. An international campaign to reopen this theological school is ongoing.

The Princes' Islands, officially just Adalar ; alternatively the Princes' Archipelago; is an archipelago off the coast of Istanbul, Turkey, in the Sea of Marmara. The islands constitute the municipality and district of Adalar within Istanbul Province. With a total land area of 11 km2 (4.2 sq mi), it is the fifth smallest district in Istanbul, and with a permanent population of 16,690 (2022), it is by far the least populous district in Istanbul.