When the old master of German historical research and historiography Leopold von Ranke congratulated the Austrian historian Alfred von Arneth on his appointment as director of the Haus-, Hof- und Staatsarchiv on August 11, 1868, he pointed out that it was “the most important archive for German history.”1 A hundred years ago access to archives in general and those of the Habsburg empire in particular was very limited. Everywhere, archival material was still subject to a strict preliminary censorship before scholars could examine it, and most holdings were not available for research purposes. Yet, in spite of all these restrictions, Ranke could with a good conscience shower the Vienna archive with words of praise. Today the abundance of the treasures of the Haus-, Hof- und Staatsarchiv, which are so generously put at the disposal of the researcher, proves Ranke's judgment to have been accurate. In many ways the archive deserves words of even higher praise than Ranke's, because it will always be a first-rate research center not only for German history but also for that of a large part of Europe. In historical significance it is now surpassed only by the Vatican archive in Rome; otherwise, it measures up well with all other archives, including those of Western Europe.2