Lên đồng (Vietnamese: [len ɗə̂wŋm], "to mount the medium", or "going into trance") is a ritual practiced in Vietnamese folk religion and the mother goddess religion Đạo Mẫu, in which followers become spirit mediums for various deities. Also known as hầu bóng ("receiving incarnations of the deities"), hầu đồng, or đồng bóng, sessions involve a number of artistic elements, such as music, singing, dance and the use of costumes. The invocation songs (Vietnamese: hát văn) used to induce a trance in mediums have been described as a "particularly noteworthy expression of the performing art of the Kinh people"; the lên đồng ritual itself is considered to be an element of Vietnam's intangible cultural heritage.
The date of lên đồng rituals are typically planned to coincide with a festival, anniversary, or the inauguration of a Mother house, although rituals may also be performed at more informal occasions. Rituals are generally held in temples, pagodas or similarly sacred locations.Votive offerings—which may range from flowers, cakes and sweets to alcohol, cigarettes and jewelry—and, on certain occasions, even Coca-Cola cans—must be purchased. Mediums generally also purchase a number of different costumes to be worn during the ritual. Before the main ritual takes place, mediums undergo several days of purifying rituals, involving abstention from sexual intercourse and eating meat.
The main ritual, which may last from two to seven hours, begins with petitions to Buddha and to the deities for permission to carry out the ritual, after which the medium seats him or herself (both men and women may act as mediums) in the middle of four assistants, whose job it is to facilitate the medium's incarnation of different deities and spirits. Specially trained musicians and singers will perform invocation songs to induce a trance in the medium, at which point he or she will be ready to incarnate different spirits.