The book, using the example of the bilingual community of Mariupol (Azov) Greeks, examines the role of language in the process of ethnic self-identification. Part of the community, the Urums, speaks Urum (one of the Turkic languages), while the native language of another part of the community, the Rumeans, is Rumean (a Greek group of the Indo-European family). The monograph is the first to undertake an analysis of the identity of this group within the framework of a constructivist approach to ethnicity, and the preservation of language is considered in the context of the linguistic loyalty of the group. Analysis of the self-awareness of the Turkic-speaking Greeks helps to better understand ethnic processes, including in stable communities with consistent characteristics. The work is based on the author’s field materials and archival sources and introduces a significant amount of new data into scientific circulation. For ethnologists, linguists, sociologists, and also everyone who is interested in ethnic processes in post-Soviet territories and the preservation of endangered languages.