Federated digital libraries are composed of distributed autonomous (heterogeneous) information services but provide users with a transparent, integrated view of collected information respecting different information sources' autonomy. In this paper we discuss a federated system for the Networked Digital Library of Theses and Dissertations (NDLTD), an international consortium of universities, libraries, and other supporting institutions focused on electronic theses and dissertations (ETDs). The NDLTD has so far allowed its members considerable autonomy, though agreements are developing on metadata standards and on support of the Open Archives initiative that eventually will promote greater homogeneity. At present, federation requires dealing flexibly with differences among systems, ontologies, and data formats. Our solution involves adapting MARIAN, an object oriented digital library retrieval system developed with support by NLM and NSF, to serve as mediation middleware for the federated NDLTD collection. Components of the solution include: 1) the use of several harvesting techniques; 2) an architecture based on object-oriented ontologies of search modules and metadata; 3) diversity within the harvested data joined to a single collection view for the user; and 4) an integrated framework for addressing such questions as data quality, information compression, and flexible search. The system can handle very large dynamic collections. An adaptable relationship between the collection view and harvested data facilitates adding new sites to the federation and adapting to changes in existing sites. MARIAN's modular architecture and powerful and flexible data model work together to build an effective integrated solution within a simple uniform framework. We present both the general design of the system and operational details of a preliminary federated collection involving several thousand ETDs in four different formats and two languages from USA and Europe.