WorldCat is an online catalog that itemizes the collections of 72,000 libraries in 170 countries and territories which participate in the Online Computer Library Center (OCLC) global cooperative. It is built and maintained collectively by the participating libraries. It contains more than 300 million records, representing over 2 billion physical and digital assets in more than 470 languages, as of January 2014. It is the world's largest bibliographic database. Source: Wikipedia
It contains the catalog records for a large number of dissertations and theses, in various formats, held in libraries around the world, though it probably does not link to the full text of these items.
Click the image above to enter the database.
Enter WorldCat by clicking the image in the box above. This will take you to the Advanced Search Screen.
On the Advanced Search screen, enter your search terms in the appropriate search fields, then look down the screen until you come to the "Narrow your search (optional)" box. In that box, find the Content menu. Pop open the Content menu and choose "Thesis/dissertations."
Then click Search. This will cause WorldCat to search only for dissertations and theses that match your search criteria.