There has been a renewed interest in psychology and anthropology in the idea of altered states of consciousness. This paper begins by examining the meaning of this term and the extent to which such experiences are reported globally. The topic of shamanism is then discussed, first with respect to its social functions, and then to what is known about its psychological aspects (which is little). Far more is known about altered states of consciousness (ASCs) as they are expressed in meditation, trance and possession, and channeling, and a number of studies are reviewed, both of individuals and of groups, which illustrate the full extent of ASCs. The following section then goes more deeply into the stimulus situations that are associated with ASCs. This brings the discussion into drugs, attempts at altering the sensorium (music and imagery in particular) and recent technological innovations, peculiar to our time, which deliberately alter our usual state of awareness through feedback principles. In the final section, what is known about the physiology of ASCs is reviewed and also the extent to which these phenomena, including shamanism, are regarded as pathological. As a great number of people across the world utilize the healing aspects of trance, possession and shamanic practices, this aspect is discussed, and lastly the adaptive functions of ASCs are considered, with their context in the history of psychology.