So you have learned the terminology and prepared your letters for archival access. Now the serious problem remains of determining where the archives you need are located and how they are organized. One of the best summaries of the organization of materials in archives was compiled by Mark Steinberg. You can find it in more detail on the library website here. The guide provides an excellent overview of the types of archives and other institutions that might have the archival materials you need.
In addition to the useful information above, contemporary archival websites (both national sites and individual archival sites) can often be very useful for locating specific information about archival holdings. This is something that was not possible to do until quite recently, but now researchers must do their best to exhaust all possible opportunities for resource perusal before embarking on a long, expensive research trip. It is important to remember that in very rare cases will actual archival materials be available online, but what is available is detailed holdings records, allowing scholars to do all their research planning from home and minimizing the time spent searching for materials while overseas.
For example, researchers interested in Russia may find much useful information at the Russian archives home site. From here, selecting the Central Fond Catalog (below) will lead to an interactive, searchable site where subject searching can provide some information about which archives are best suited for the planned research agenda.
In addition, one can examine archival holdings at a large number of Russian institutions through the online archival guides page, also available from the home site of the Russian archives.
Outside Russia, many other national archival systems are increasingly emphasizing digital, online access for holdings and resources. For example, the Croatian State Archive has organized a portal from which one can see the general holdings of the regional state archives as well as the central archive in Zagreb.
Perhaps no other national library system has been more active in digitizing and making library and archival holdings openly available online than that of Hungary. As part of the their national archive site available from the home page of this guide, patrons can find the Magyar Archivum site, which has organizes (in English) a wealth of archival holdings information.
What these examples should illustrate is that this is a rapidly evolving field. The amount of material across the region that is being made accessible online in digital format is so vast that any guide to the subject becomes quickly out of date. Researchers should spend some time and familiarize themselves with the homepages of the national archives, because they might discover something incredibly useful that they had not planned on.