Glossolalia is the most-studied religious experience of the twentieth century and the most divisive. Participants describe fluent speech that outruns deliberate control, usually in a congregation praying aloud together, and report relief and warmth rather than dissociation; neuroimaging studies find reduced frontal activity consistent with that self-report, and linguists find the output lacks the structure of natural language. The Azusa Street revival in Los Angeles in 1906 made it the marker of Spirit baptism for the emerging Pentecostal movement, and classical Pentecostal doctrine treats it as the initial physical evidence — a position many charismatics inside historic denominations do not hold. Cessationists argue the gifts described in Acts and Corinthians ended with the apostolic age. Around half a billion people are on the Pentecostal side, which makes this the largest disagreement about the nature of religious experience inside any tradition.