University library of Heidelberg: Catalogs of libraries of the University Heidelberg and associated institutes.

 Catalog of the library at the Max-Planck-Institute for Astronomy.

 Deutsche Nationalbibliothek: The German National Library is the central archival library and national bibliographic center for the Federal Republic of Germany.

 Karlsruhe Virtual Catalogue: Karlsruhe Virtual Catalog KVK - more than 500 million books and serials in library and book trade catalogs worldwide.

 Südwestdeutscher Bibliotheksverbund: This bibliographic database references all kinds of media of more than 1000 libraries in Baden-Wuerttemberg, Saxony, Saarland, and Rhineland-Palatinate. It contains 47 million holding records for about 12 million titles of mainly scientific literature. These comprise 1.2 million holding records of 350.000 journals.

Technical Information and University Library Hannover: In its role as the German National Library of Science and Technology and the related basic sciences, the TIB/UB has spent decades successfully meeting the challenge of being an innovative and competent information provider.

 Library of Congress: The American National library.