Spirit possession and trance phenomena (PTP) are common dissociative occurrences throughout the world, both as functional and dysfunctional manifestations. The chapter provides examples of non-pathological (the most common) and pathological possessions, provides criteria to distinguish between spirit possession and trance, and between both of them from other alterations of consciousness. A section then discusses distinguishing features of non-pathological and pathological PTP, including psychological and sociocultural variables. After surveying different explanatory frameworks for PTP, the chapter concludes with areas deserving particular research and clinical attention.