The phenomenon of “possession” continues to elude satisfactory explanation because of the ambiguity of its nature. It belongs to one of those marginal zones where beliefs and rites are allied in the closest possible way to still obscure psychological mechanisms. We know that the phenomenon dates from antiquity, and that in numerous so-called primitive societies it is one of the means by which the faithful communicate with the supernatural. Our object is to offer a contribution to the clarification of this ambiguity by an analysis of the forms and functions of the trance in voodoo. This term, originating in Dahomey (in Fon, a voodoo is a god or spirit), denotes the ensemble of religious beliefs and practices observed on the fringes of Catholicism by the common people of Haiti. It is a syncretic religion in which elements from West Africa predominate, although other African cults have left their mark; however, this religious amalgam was impregnated from the beginning with Catholic beliefs and rites. The practice; of voodoo is not limited to Haiti. Under such names as macumba, candomblé, santeria, it is found in Brazil and in Cuba, where the Negroes, closer to the period of slavery, have conserved African traditions more completely than in Haiti. Observations made in Brazil can help to explain certain aspects of possession which, in Haiti, took an aberrant form.