Encephalopathy with Electrical Status Epilepticus during Sleep (ESES) was described almost 50 years ago; however its clinical spectrum and its boundaries, the diagnostic criteria, the pathophysiology and the therapeutic management are still debated. In the last years, clinical data, neurophysiological and imaging investigations, as well as genetic studies have renewed the interest on ESES. In addition, experimental findings from sleep research have opened fascinating perspectives on some possible pathophysiological mechanisms involved in this condition. These issues are presented and discussed in this book by clinicians, neurophysiologists, sleep physiologists and geneticists. They all have been working on ESES with the aim to provide an updated overview of this special syndrome in the light of recent research.