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.

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Digital libraries have the potential to not only duplicate many of the services provided by traditional libraries but to extend them. Basic finding aids such as search and browse are common in most of today's digital libraries. But just as a traditional library provides more than a card catalog and browseable shelves of books, an effective digital library should offer a wider range of services.Using the traditional library concept of special collections as a model, in this paper we propose that explicitly defining sub-collections in the digital library-virtual collections-can benefit both the library's users and contributors and increase its viability. We first introduce the concept of a virtual collection, outline the costs and benefits for defining such collections, and describe an implementation of collection-level metadata to create virtual collections for two different digital libraries. We conclude by discussing the implications of virtual collections for enhancing interoperability and sharing across digital libraries, such as those that are part of the National SMETE Digital Library.