There are a lot of different citation systems; every discipline has its own favourite systems, so ask your teacher before choosing the one you want to use. Be careful not to mix different systems. Once you have chosen one, stick to it for all your paper.

The citation styles are divided in three main families:

• Author/date systems: this family of citation styles uses in-text references putting author name and publication year in brackets, while in the final bibliography there are complete references in alphabetical order. Belong to this family APA style and Harvard style, most used in the social sciences.
• Documentary/note styles: in this kind of citation styles, citations must be placed either in footnotes or endnotes, sequentially numbered in the text. At the end of the text, there is a bibliography listing all the references. Belongs to this family the Chicago style, mainly used in the humanities.
• Numeric styles: in this styles, citations are only numbers in square brackets; each number referes to a complete reference in the final bibliography. Belong to this family the IEEE style, used in the engineering, and the Vancouver style, used in medicine.