The original database providing citation searching, actually began as a printed index: Science Citation Index. Everyone knows that s/he could find additional relevant papers by looking at the references, (the bibliography) at the conclusion of papers, but this academic searching device was developed into an indexing service by Dr. Eugene Garfield, who founded the Institute for Scientific Information (ISI) in Philadelphia in the 1960s. He compiled all the bibliographies from all the papers in major journals of all scientific fields. Science Citation Index has become Web of Science — or Web of Knowledge — which also includes the humanities and social sciences. One can choose the “Cited Ref Search”, or run a search and click on the number that follows “Times Cited” for the paper in question.
Elsevier has developed a more transparent version of citation searching in Scopus. Compose and run a search, then combine all relevant references in a “Cited by” search, or simply click on the number in the “Cited by” column. The Scopus database goes back reliably to 1996, however, Web of Knowledge goes back to 1900. IEEE Xplore has just announced that it, too, has citation searching. Click on the Search “button” on the orangy-brown background. In the drop-down menu, one choice will be: Citation Search. Fill out as much of the search form for the level of specificity that you want. Keep in mind that the retrieval will only be the papers that have been cited in this IEEE collection.