It is commonly understood that sleep in some form or another is an almost universal phenomenon among animals, including humans. However, the specific reasons why we need sleep are not as clear. The articles in this volume explain what sleep is, the benefits it has on the mind and the body, and its role in cognitive and biological functions. It examines disorders that disturb normal sleep and what these disorders tell us about our need for sleep. It also considers what researchers have yet to discover about sleep and the methods they are using to try to improve our understanding.