The distributed, project-oriented nature of digital libraries (DLs) has made them difficult to evaluate in aggregate. By modifying the methods and tools used to evaluate traditional libraries’ content and services, measures can be developed whose results can be used across a variety of DLs. The DigiQUAL protocol being developed by the Association of Research Libraries (ARL) has the potential to provide the National Science Digital Library (NSDL) with a standardized methodology and survey instrument with which to evaluate not only its distributed projects but also to gather data to assess the value and impact of the NSDL. Categories and Subject Descriptors J.4 [Computer Applications]: Social and Behavioral Sciences -economics, psychology, sociology General Terms Measurement, Human Factors Keywords Digital Library Service quality; Evaluation; Libraries; Educational Digital Libraries