Google Scholar is a publicly available online search engine that indexes the full text or metadata of academic literature from a variety of publication formats and fields. It is available to anybody who has access to the internet. The Google Scholar index, which was first made available in beta in November 2004, includes the majority of peer-reviewed online academic journals and books, conference papers, theses and dissertations, preprints, summaries, technical reports, or other scholarly literature, including court opinions and patents. The index is updated daily.
Google Scholar makes use of a web crawler, sometimes known as a web robot, to find files that should be included in the search results list. A number of requirements must be met before material may be included in Google Scholar's indexing system. An previous statistical estimate published in PLOS One, which used a Mark and recapture approach, projected that around 80–90 percent of all papers published in English were covered, with an estimated total of 100 million articles covered in the sample. This estimate also established how many papers were freely accessible on the internet, which was based on the previous estimate. For not screening publications and include predatory journals in its database, Google Scholar has been chastised by the scientific community.
When Google scanned the holdings of the University of Michigan Library and other libraries for use in Google Books and Google Scholar, copies of the scans were preserved, and these copies were used to build the HathiTrust Digital Library, which is now available to the public.