The Convention Concerning the Exchange of Greek and Turkish Populations (Greek: Σύμβαση για την Ανταλλαγή των ελληνικών και τουρκικών Πληθυσμών, romanized: Symvasi gia eis Antallagi ton ellinikon kai tourkikon Plithysmon, Turkish: Türk ve Yunan Nüfuslarının Mübadelesine İlişkin Sözleşme), also known as the Lausanne Convention, was an agreement between the Greek and Turkish governments signed by their representatives in Lausanne on 30 January 1923, in the aftermath of the Greco-Turkish War of 1919–1922.
With respect to the Muslims of Greece the treaty reflected Ottoman conceptions of 'nationality' in that their actual ethnic origins was superseded by religious affiliation.
Also many historic cases of Pontic Greeks from northeastern Anatolia and the Trans-Caucasus region who had converted to Islam and adopted the Turkish-language and national identity were simply classified for the purposes of the convention as 'Turks'.
However, large numbers from that Pontic Greek community had remained Crypto-Christians into the late Ottoman period, before reverting to their ancestral Christian Orthodox faith following the 1828 Russian occupation of Erzurum and Gümüşhane, when they joined the invading forces and then followed the Russian Imperial Army back into Georgia and southern Russia upon its withdrawal.
It was registered in the League of Nations Treaty Series on 27 January 1925.