Usually researchers find more sources by looking at the footnotes in an article or book, but these will always be older than the publication you have in hand. Citation indexes like the Web of Science (which includes sections for the Social Sciences, Arts and Humanities, and Science) are set up to search for sources cited in the footnotes of journal articles. This allows you to find newer articles which cite the books and articles you know are key for your topic. By relying on connections between authors and by moving forward in time, citation searching can open up new avenues