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 split 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.