Google Scholar (GS) is a freely-accessible Web search engine that inexes the full text of scholarly literature across an array of publishing formats and disciplines. Released in beta in November 2004, the GS index includes most peer-reviewed online journals of the world's largest scientific publishers. It is similar in function to the freely available Scirus from Elsevier, CiteSeer, and getCITED. It is also similar to the subscription-based tools, Elsevier's Scopus and Thomson ISI's Web of Science. GS nonetheless claims to cover more websites, journal sources and languages. Its advertising slogan — "Stand on the shoulders of giants" (paraphrasing a quotation from Isaac Newton) — is a nod to the scholars who have contributed to their fields over the centuries, providing the foundation for new intellectual achievements.