Hillegass and his wife, Catherine, started the business in their basement at 511 Eastridge Drive in Lincoln, with sixteen [[William Shakespeare]] titles. By 1964 sales reached one million Notes annually. CliffsNotes now exist for hundreds of works. The term "Cliff's Notes" has now become a [[Generic trademark|proprietary eponym]] for similar products.
[[International Data Group|IDG Books]] purchased CliffsNotes in 1998 for $14,200,000.2 million. [[Wiley (publisher)|John Wiley & Sons]] acquired IDG Books (renamed Hungry Minds) in 2001. In 2011, CliffsNotes announced a joint venture with [[Mark Burnett]], a TV producer. This would be a series of 60-second video [[study guide]]s of literary works.<ref name=marketplace>{{Cite web|title=CliffsNotes Goes Digital|url=http://marketplace.publicradio.org/display/web/2011/03/10/pm-cliffsnotes-goes-digital//|access-date=March 10, 2011|publisher=American Public Radio|archive-url=https://web.archive.org/web/20110727192539/http://marketplace.publicradio.org/display/web/2011/03/10/pm-cliffsnotes-goes-digital//|archive-date=July 27, 2011|url-status=dead}}</ref> In 2012, CliffsNotes was acquired by [[Houghton Mifflin Harcourt]].<ref name=about>{{Cite web|title=About CliffsNotes|url=https://www.cliffsnotes.com/discover-about|website=CliffsNotes|access-date=June 20, 2015}}</ref> In 2021, CliffsNotes was acquired by [[Course Hero]].<ref>{{Cite web|title=Online education unicorn Course Hero buys CliffsNotes|publisher=Silicon Valley Business Journal|access-date=August 21, 2021|url=https://www.bizjournals.com/sanjose/news/2021/08/19/course-hero-buys-cliffsnotes.html}}</ref>
