A great deal of highly specialized scholarship is never formally published and appears only as a thesis or dissertation. While recent U.S. and Canadian dissertations and theses are easy to locate -- thanks to centralized control at UMI/Proquest -- older dissertations, master's theses, and foreign dissertations can be difficult to find. Even when you can identify a dissertation that you want to read, it is not always possible to obtain a copy. Keeping that in mind, here are some tools that will help you identify and locate copies of theses and dissertations from U.S. and non-U.S. colleges and universities.
The University Archives holds copies of most Princeton dissertations: Access to Ph.D. Dissertations. For dissertations written from 1989 to the present, search the library catalog for "Princeton University. Dept. of History" as author; for earlier, try a keyword search for "history and thesis and princeton." A card file and a local database at Mudd may help in locating theses that are obscure or missing in the Main Catalog.
The American Historical Association maintains a list of dissertations in progress at http://www.historians.org/pubs/dissertations/. The list is provided by history departments annually, so it may not be complete, but it's a useful starting point.