Flexible digital library systems need to be able to accept, or “import,” documents and metadata in a variety of forms, and associate metadata with the appropriate documents. This paper analyzes the requirements of the import process for general digital libraries. The requirements include (a) format conversion for source documents, (b) the ability to incorporate existing conversion utilities, (c) provision for metadata to be specified in the document files themselves and/or in separate metadata files, (d) format conversion for metadata files, (e) provision for metadata to be computed from the document content, and (f) flexible ways of associating metadata with documents or sets of documents. We argue that these requirements are so open-ended that they are best met by an extensible architecture that facilitates the addition of new document formats and metadata facilities to existing digital library systems. An implementation of this architecture is briefly described.