This chapter provides an overview of idiopathic hypersomnia together with a discussion on the nosological limits of the condition. It presents the history, clinical features, diagnostic procedures, treatment and differential diagnosis of idiopathic hypersomnia. All efforts to clarify demographics, pathophysiology and treatment have been hampered by the nosological uncertainty on the borders of idiopathic hypersomnia. The latest International Classification of Sleep Disorders confirmed the distinction of two forms of idiopathic hypersomnia and used a new wording of idiopathic hypersomnia with long sleep time and idiopathic hypersomnia without long sleep time. The diagnosis of idiopathic hypersomnia with long sleep time is mainly based on clinical features and on the absence of associated symptoms such as snoring or cataplexy. However, polysomnography and the multiple sleep latency test (MSLT) are indispensable to rule out other conditions. Further research is necessary to specify the limits of idiopathic hypersomnia with and without long sleep time.