Google Scholar is a freely accessible web search engine that indexes the full text of scholarly literature across an array of publishing formats and disciplines. Released in beta in November 2004, the Google Scholar index includes most peer reviewed online journals of Europe and America's largest scholarly publishers, plus scholarly books and other non-peer reviewed journals. It is similar in function to the freely available CiteSeerX and getCITED. It is also similar to the subscription-based tools, Elsevier's Scopus and Thomson ISI's Web of Science. Its advertising slogan – "Stand on the shoulders of giants" – is taken from a quote by Isaac Newton and is a nod to the scholars who have contributed to their fields over the centuries, providing the foundation for new intellectual achievements.