The accelerating rate of scientific and technical discovery, typified by the ever-shortening time period for the doubling of information - currently estimated at 18 months [1] - causes new topics to emerge at increasing speed. Libraries have a hard time just cataloguing the large amount of produced documents. Scientists and practitioners who must read and process relevant documents are in need of new tools that can help them to identify and manage this flood of information. Visual Interfaces to digital libraries apply powerful data analysis and information visualization techniques to generate visualizations of large document sets. The visualizations are intended to help humans mentally organize, electronically access, and manage large, complex information spaces and can be seen as a value-adding service to digital libraries. This introductory chapter motivates the design and usage of visual interfaces to digital libraries, reviews diverse commercially successful systems, discusses major challenges, and provides an overview of the chapters in this book.