  It may be difficult to imagine today’s world without search engines. Which high school student has not used a Web search engine to query about some topic or subject? Of course, it is quite possible that many Internet users, both young and old, do not consciously distinguish between the search engines they use and Web browsers that now also typically include search engines as a feature within their user interface. But virtually all Internet users have come to expect and depend on the instantaneous results they receive in response to their various search queries. While there is no shortage of definitions of “search engine,” none has been accepted as the standard or universally agreed upon definition. For purposes of this entry, however, the definition of a (Web) search engine, put forth by Halavais (2009, 5–6), is “an information retrieval system that allows for keyword searches of distributed digital text.” We note that this definition includes some important technical terms and concepts that, in turn, need defining and further elucidation. Our examination of key technical concepts underlying search engines is intended to provide a useful context for our analysis of the ethical implications. In this sense, Blanke (2005, 34) is correct that an adequate analysis of the ethical aspects of search engines “requires knowledge about the technology and its functioning.”