The chapter focuses on the early Christian interpretations of “speaking in tongues” (glōssais lalein), the most spectacular linguistic phenomenon attested in the New Testament. The description of the Pentecostal events (Acts 2) and Paul’s exhortations (1 Cor. 12–14) became an important point of reference in Christian discussions about languages and religious identity. The second- and third-century authors either presented the phenomenon as ecstatic speaking with an uncertain degree of intelligibility or simply quoted biblical passages without any explanation. Explicit statements that the gift of tongues was a miraculous ability to talk in foreign languages that enabled apostles to preach abroad (xenolalia) are dated to the fourth century and attested in Greek, Syriac, and Latin texts. Simultaneously, in the fourth century, the alternative idea that “speaking in tongues” refers to angelic languages decreased in popularity. As time passed, different Christian traditions and authors developed their own peculiarities in interpreting “speaking in tongues.” The chapter demonstrates various ways in which otherness of tongues may have been understood; and that xenolalia is not so much a default interpretation, but a way to channel the growing concerns about foreign languages and their speakers – a way that became especially needed in fourth-century Christianity.