Outlines the actual and potential uses of web services in a library context and recommends a set of best practices in support of interoperable digital library services. Included in this document is a discussion of the document service interface, looking at four model types. Best practices are explained in the areas of HTTP caching, filtering of user input, reuse of output formats, security, and throttling. Typical output formats used in web services-DTD, XML schema, RDF, Relax NG, and DSD-are described. An appendix provides an overview, for those new to web services, of the typical types of services used in a library context: discover, locate, request, deliver, and common services. The appendix also includes a brief introduction to interoperability issues.