• Scopus: A broad, general database of peer-reviewed literature across most scientific disciplines, and some of the social sciences. In addition to articles, Scopus includes book chapters and conference proceedings. Most of the content here includes the full text.
• Web of Science: Another broad, general database - strongest in the sciences, but it includes the humanities too. Note that Web of Science is an abstract database, which means it indexes abstracts and other information from academic articles to let you know what has been published in a research area.
• Articles+: Covers all subjects and many different formats (journal and magazine articles, news coverage, book reviews, dissertations), with a mix of peer-reviewed and popular sources. A good place to start when you're not sure where to begin - but be aware that the size of the database (millions of records) means there may be extra steps to find exactly what you need.

(AIDS Collection (MS 1834). Manuscripts and Archives, Yale University Library.)