Foreword Barry Jacobs once memorably commented that the problem in understanding the role of serotonin in the brain was that it was implicated in virtually eve- thing but responsible for nothing [1]. Serotonin (5-HT) is defnitely implicated in the regulation of sleep but what does it do? Once seen as a sleep-promoting molecule, Jacob’s own work, lucidly described in this volume, radically changed our view. In fact, serotonin neurons are most active during waking; their - tivity diminishes with sleep onset and disappears during rapid eye movement (REM) sleep. Serotonin, therefore, is intimately linked to the sleep-wake cycle, but what is its role, what mechanisms are involved and what are the impli- tions for sleep medicine and psychiatry? Making progress with these diffcult questions requires a “translational” approach, which is a research paradigm where both laboratory and clinical neuroscience inform and guide each other with the ultimate aim of improving understanding and treatment of medical conditions. This volume is a superb example of translational research, where leading basic and clinical scientists - tegrate molecular, neuropharmacological and systems approaches to illuminate the reciprocal interactions of serotonin neurons and the mechanisms involved in sleep and circadian regulation. Serotonin is an ancient chemical mediator preserved through at least 500 m- lion years of evolution. In invertebrate animals such as mollusks and leeches, 5-HT cells are distributed throughout the nervous system among the various ganglia.