Google Scholar is a freely-accessible web search engine by Google that indexes the full-text of scholarly literature across a very wide array of publishing formats and disciplines.
Explore the ISI Web of Science database with multidisciplinary coverage of over 10,000 high-impact journals in the sciences, social sciences and arts and humanities, as well as over 120.000 conference proceedings.
PubMed by the U.S. National Library of Medicine includes over 18 million citations for biomedical articles back to the 1950s. PubMed includes links to the full text and related resources.
UniCat is a union catalogue of Belgian libraries. It's the successor of the CCB (Collectieve Catalogus Begië). It currently holds some 14 million records from the main Belgian university libraries and the Royal Library.
Antilope is the Belgian union catalogue of periodicals. You can search it to find a library holding the specific volumes/issues you are looking for.
WorldCat lets you search the collections of libraries in your community and thousands more around the world. OCLC network grows every day.