If you need primary literature (scientific studies or journal articles) the best tool to use will depend on the subject you are researching. Each search engine or database has particular characteristics you should keep in mind when selecting one to search, such as:

• What subjects does it cover?
• Does it include different types of publications, such as scholarly and popular articles?
• What dates of publication does it cover?

For example, PubMed and the Public Library of Science: Medicine are good databases to search for articles that pertain to medicine or human biology. If, however your topic is photosynthesis, BIOSIS Previews or the Science Citation Index may be better sources to search.