ProQuest Dissertations and Theses contains records for most North American doctoral dissertations. To locate dissertations of interest, search by keyword. Select More Search Options or use the Advanced search interface to search for terms in specific fields (Author, School, etc.). Note that some fields (Abstract, Advisor, Committee Member, Department) will not work for dissertations produced before a certain date. Most titles submitted since 1997 are available online.
The Theses Canada Portal provides additional information about Canadian theses.
Dissertations and theses from institutions outside the United States and Canada can sometimes be challenging to obtain. The Center for Research Libraries has a large collection of foreign dissertations. We borrow material from them, and in many cases interlibrary loan staff will ask CRL to acquire material they do not hold. See the CRL guide to dissertations for additional information.
While ProQuest Dissertations & Theses and the CRL Guide to Dissertations are good starting points, many of the Library's subscription databases contain records for dissertations. Along with the records for books and other material, WorldCat contains records for dissertations or theses held by member libraries. Most of the major subject-specific indexes (such as PsycINFO, SciFinder Scholar, or Sociological Abstracts) will also include citations for dissertations. Some of the Library's research guides mention additional sources.
The Library also has some print sources and bibliographies which you may use to locate dissertations. Locate these sources by searching the Library Catalog using the subject term Dissertations, Academic in either the subject keyword search or the Begins with subject search option. If you are looking for dissertations from a specific country, you can add the country into your search terms (i.e. Dissertations, Academic -- Mexico).
The ALA Guide to Reference lists other sources in the "Dissertations" section.