Google Scholar seems to have the entry for almost every thesis I can find in google.com. However, I do not know how to search specifically for theses alone.
For instance, to find a PhD thesis written specifically for a topic, I Google the following
"Topic Name" PhD Thesis
The results are not bad, but it would be nicer to have a result as provided by Google Scholar.
I have had some success by doing a similar type of search in Google Scholar. For example, if I want to search for dissertations related to convex optimization, the following search terms in Google Scholar
"convex optimization" dissertation site:edu
returns many useful results. In the above example, the search is restricted to those institutions that are archiving their dissertations on the .edu domain. Obviously, you can alter the domain, if need be.
You can also substitute "thesis" for "dissertation", but Google Scholar seems to be smart enough to give results for either, no matter which one you use.
