PubMed provides free access via the Internet to more than 23 million records, of which over 19 million are from the MEDLINE database of journal articles. PubMed also provides access to other databases, such as the NCBI Bookshelf. To perform a basic search, you can simply enter the search terms or the concept that you are looking for in the search box. However, taking care to clarify your key concepts may save much time later on, because a non-specific search is likely to produce an overwhelming number of result hits. One way to make your search more specific is to specify which field you want to search using field tags. By default, the results of a search are sorted by the date added to PubMed and displayed in summary format with 20 result hits (records) on each page. In summary format, the title of the article, list of authors, source of information (e.g., journal name followed by date of publication, volume, issue, pages) and the unique PubMed record number called the PubMed identifier (PMID) are shown. Although information is stored about the articles, PubMed/MEDLINE does not store the full text of the papers themselves. However, PubMedCentral (PMC) stores more than 2.8 million articles (roughly 10% of the articles in PubMed) and provides access to them for free to the users.