Federated digital libraries are composed of distributed, autonomous, and often heterogeneous information services but provide users with a transparent, integrated view of collected information. 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). Federation requires dealing flexibly with differences among systems, ontologies, and data formats while respecting information sources’ autonomy. Our solution involves adapting the object-oriented digital library system MARIAN to serve as mediation middleware for the federated NDLTD collection. Components of the solution include: 1) the use and integration of several harvesting techniques; 2) an architecture based on object-oriented ontologies of search modules and metadata; 3) reconciliation of 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, flexible and efficient search, and scalability.