In this article, the issue of shamanism and altered states of consciousness (ASC) is reviewed from a cross-cultural and multidisciplinary perspective. It is suggested that in spite of considerable differences in the uses of these terms, there are conceptual and empirical grounds for distinction among different types of trance practitioners. The authors argue that shamanism is a cultural adaptation of hunting and gathering societies to the biological potential for ASC, and that the specific nature of that manifestation changes as societies become more complex. The role of ASC in understanding shamanic phenomena, the roots of religious experience, and the modem manifestations of the potential for trance are examined. Western cultural avoidance of ASC has inhibited understanding of these phenomena, and has prevented an integration of shamanistic and trance perspectives into the understanding human of psychology, consciousness, and knowledge of the world.