The aging process is associated with characteristic changes in subjective and objective sleep. The chapter reviews the basic brain mechanisms underlying the global changes in behavioral state from waking, to non-rapid eye movement (NREM), and to rapid eye movement (REM) sleep, as well as the core brain structures that play roles in the generation and modulation of these brain states. Changes in REM sleep in the elderly may be related to the brain structures that either generate or maintain REM sleep. These structures include the pontine reticular formation, the basal forebrain and hypothalamus, the amygdala, and anterior paralimbic structures such as the anterior cingulate cortex and the medial pre-frontal cortex. In large part, preclinical studies have focused on either the primary nuclei that generate REM sleep or that are functional in mediating the cortical arousal as it relates to either REM sleep or waking.