Evaluation of digital libraries (DLs) is essential for further development in this area. Whereas previous approaches were restricted to certain facets of the problem, we argue that evaluation of DLs should be based on a broad view of the subject area. For this purpose, we develop a new description scheme using four major dimensions: data/collection, system/technology, users, and usage. For each of these dimensions, we describe the major attributes. Using this scheme, existing DL test beds can be characterised. For this purpose, we have performed a survey by means of a questionnaire, which is now continued by setting up a DL meta-library.