Managing the organizational and software complexity of a comprehensive digital library presents a significant challenge. Different library collections each have their own distinctive features. Different presentation languages have structural implications such as left-to-right writing order and text-only interfaces for the visually impaired. Different media involve different file formats, and-more importantly-radically different search strategies are required for non-textual media. In a distributed library, new collections can appear asynchronously on servers in different parts of the world. And as searching interfaces mature from the command-line era exemplified by current Web search engines into the age of reactive visual interfaces, experimental new interfaces must be developed, supported, and tested. This paper describes our experience, gained from operating a substantial digital library service over several years, in solving these problems by designing an appropriate software architecture.