The discovery of the close association between rapid eye movement (REM) sleep and dreaming and development of sleep laboratory techniques ushered in a new era in the study of dreams. For the first time, direct and systematic investigation could be made of such topics as the occurrence, qualities, recollection, and childhood development of dreaming. Experimental methodologies permitted investigation of the responsiveness of dreams to external stimulation and the effects of deprivation of REM sleep. Much effort was devoted to searching for parallels between physiological aspects of REM sleep and characteristics of associated dreams, with modest results. The leading theory of dreaming in the early decades of this research was the psychoanalytic, which views dreams as highly meaningful reflections of unconscious mental functioning. With developments in understanding of the neurophysiology of REM sleep, new theories of dreaming were proposed. The most prominent, the activation-synthesis hypothesis, derived its view of dreaming directly from the neurophysiology of REM sleep, in particular the role of the brain stem, and in its original form regarded dreams as not essentially meaningful. Further developments in neurobiological research, including lesion and brain imaging studies, have established a clearer view of the functional neuroanatomy of REM sleep and dreaming. To what degree, and in what way, implications can be drawn from these findings for the psychology of dreaming is controversial. Some more recent theories of dreaming emphasize an adaptive function related to emotion and a role in learning and memory consolidation.