In general, especially when you are writing about science or technology, current events, or emerging trends, you should consult the most up-to-date sources available on your subject. The date of publication for articles you locate should be indicated in your search results. For a print book, look for the copyright date on the copyright page (usually on the back of the title page); for an e-book, look for the copyright date at the beginning or end of the electronic file. If your source is a Web site, consider when it, and the content within it, was last updated (often indicated at the bottom of the Web page or home page).

You may also need older, “classic” sources that establish the principles, theories, and data on which later work is based and may provide a useful perspective for evaluating other works. To determine which sources are classics, note the ones that are cited most often in encyclopedia articles, lists of works cited or references, and recent works on the subject. You may also want to consult your instructor or a librarian to help you determine which works are classics in your field.