In this article the authors propose, on the basis of a review of the evolutionary and neuropsychological study of REM sleep, that REM sleep functions to promote attachment and that in the mature state it may promote sexual pair bonding and serve related compensatory functions. The attachment hypothesis is consistent with known psychobiologic correlates of REM sleep, with classical psychoanalytic theory regarding dreams, and with evidence from research on attachment. The authors argue that this hypothesis leads to a new understanding of the role of repression and the dream work, and has broad implications for psychopathology research. They argue that although many in the cognitive and neural sciences have largely dismissed S. Freud's (1900/1953) theorizing on dreams, there is important complementarity when it is evaluated through the lens of the attachment hypothesis.