Searching German dissertations
You will find German dissertations in the online catalogue of the German National Library. You can limit your search by using the filter "Hochschulschriftenvermerk" and searching for "diss", "habil" or the place of the university.
Searching European dissertations
DART-Europe E-theses Portal lists online dissertations from Europe.
Searching dissertations internationally
The project Networked Digital Library of Theses and Dissertations (NDLTD) claims to build a worldwide virtual library of bachelor theses and dissertations.