After a decade of research and development, digital libraries are becoming operational systems and services. This paper summarizes some of the challenges required for that transition. Digital libraries as systems are converging with digital libraries as institutions, particularly as we consider the service aspects. They are enabling technologies for applications such as classroom instruction, information retrieval, and electronic commerce. Because usability depends heavily upon context, research on uses and users of digital libraries needs to be conducted in a wide array of environments. Interoperability and scaling continue to be major issues, but the problems are better understood. While technical work on interoperability and scaling continues, institutional collaboration is an emerging focus. Concerns for an information infrastructure to support digital libraries is moving toward the concept of “cyberinfrastructure,” now that distributed networks are widely deployed and access is becoming ubiquitous. Appropriate evaluation methods and metrics are requirements for sustainable digital libraries that have received little attention until recently. We need to know what works and in what contexts. Evaluation has many aspects and can address a variety of goals, such as usability, maintainability, interoperability, scalability, and economic viability. Lastly, two areas that have received considerable discussion elsewhere are noted -digital preservation and the role of information institutions such as libraries and archives.