There is a critical, bidirectional relationship between sleep and cognition, yet the literature in this area is scattered and it is surprisingly difficult to find a comprehensive overview of the basic principles, latest discoveries, and outstanding challenges. This issue of Progress in Brain Research is spilt over 2 volumes bringing together cutting-edge research on the topic in the basic, clinical and applied sciences. The 2 volumes review current knowledge and understanding, provide a starting point for researchers and practitioners entering the field, and build a platform for further research and discovery. Leading authors review the state-of-the-art in their field of investigation, and provide their views and perspectives for future research. Chapters are extensively referenced to provide readers with a comprehensive list of resources on the topics covered. All chapters include comprehensive background information and are written in a clear form that is also accessible to the non-specialist.