After a methodological foreword to the sources'acquisition, the author describes the history of the National Library of Medecine (NLM), whose aim was (and is) to gather, to classify and to publish biomedical bibliographical references through the information technologies which have been developing since 1818 till nowadays from hardcopy into MEDLINE via Internet. The author explains the concept of MeSH (Medical Subject Heading) and describes the structure of the Thesaurus MeSH: a vocabulary containing all the subjects which are used for the articles'indexing. The MeSH Browser has got all the MeSH describers which are in the hardcopy of the Medical Subject Heading, with the relative notes and the abbreviations of the corresponding subheadings: that is a classification of subheadings representing a particular aspect of the describer to which they refer. Finally, the author describes how the MEDLINE and PubMed work: the former is available through the electronic subscription to CSA (Cambridge Scientific Abstract), the latter is free and available on the NLM website. Through the several search options (free search, advanced search with boolean operators, field search or subject search etc...) one can find the selected bibliographical references in the above mentioned databases, which provide abstracts of articles and sometimes the possibility of reading the articles in full text, thanks to the link to the journals' websites.