The drive for energy efficiency arising from a growing awareness of the dangers of climate change is leading to new approaches to library design. The article explores the interaction between library architecture and sustainability by reviewing the many complex strategies employed for saving or recycling evermore scarce natural resources. Although the focus is upon energy consumption, the article also investigates how concepts of comfort and climate are changing under the impact of environmental sustainability. These topics are discussed mainly in the context of public and university libraries built over the past decade, with some reference to library designs still awaiting construction. The methodology is developed primarily through comparative case studies, exploring the approach to environmental sustainability under a number of headings drawn from widely employed environmental assessment methods such as BREEAM (Building Research Establishment Environmental Assessment Method), commonly employed in the United Kingdom and much of Europe; and LEED (Leadership in Energy and Environmental Design), the assessment tool frequently used to anticipate the environmental credentials of projects still on the drawing board in the United States and Asia. In many ways the architectural approach to the twenty-first century library is returning to the roots of the modernist library found in Scandinavia with its emphasis upon high levels of daylight, natural materials, social harmony, and contact with nature.