This chapter reviews functional neuroimaging studies in animals and humans aimed at better understanding the peculiar cerebral mode. It presents evidence that brain activity during rapid eye movement (REM) sleep is influenced by previous experience, suggesting the participation of REM sleep in memory consolidation. Functional neuroimaging research specifically devoted to the characterization of dream correlates has been conducted only during REM sleep. Indeed, mentation during REM sleep is more abundant, vivid, and story-like and hence more detailed dream reports can be obtained from REM than from slow-wave sleep. Motor behavior and movements probably activate motor-related brain areas during REM sleep. A growing body of data indicates that patterns of neural activity prevailing during sleep support offline processing of newly acquired information. The chapter concludes with comments on the difficulty in interpreting functional imaging of REM sleep in terms of neural correlates of dreaming.