Be sure to check with your professor to see which style is appropriate for your class.  

AAS (The American Astronomical Society) is the standard citation style used for astronomy writing (in some physics courses).

Guidelines for AAS style are available on the AAS website.

ACM (Association for Computer Machinery) is the standard citation style used in computer science.

The ACM World Style guide has double and single column formatting assistance, as well as in-text citation.

ACS (The American Chemical Society) is the standard citation style used in chemistry courses.

Williams College has a helpful quickguide to ACS style available online.

AIP (The American Institute of Physics) is the standard citation style used in physics and some engineering courses.

A pdf form of the 4th edition of the AIP style guide is available on Kenneth Hanson’s website. 

APA (American Psychological Association) style is often used in psychology and other social science courses.

Purdue OWL’s APA Style Guide Onlineaddresses questions about citations, reference lists, stylistics, and formatting tables and figures.

ASA (American Sociological Association) is commonly used in sociology courses.

Purdue OWL also has an ASA Formatting and Style Guide.

CMS (The Chicago Manual of Style, or “Chicago”) is often used in humanities courses and in the social sciences.

Purdue OWL has a Guide to Chicago Style that focuses on the Notes & Bibliography (NB) system commonly used in the Humanities.

They have a Sample CMS NB paper available, as well.

Williams College Library has a Guide to Chicago Style that focuses on the Author-Date system generally used in the Social Sciences and Sciences. 

Purdue OWL also provides a Sample CMS Author-Date paper.

The Chicago Manual of Style website has a condensed version available for free. 

CSE (Council of Science Editors) style is sometimes used in biology courses.

The University of Wisconsin-Madison has helpful Overview of CSE Documentation Systems.

MLA (Modern Language Association) is frequently used in English and modern language courses. 

Purdue OWL’s MLA Formatting and Style Guide has useful information and models for formatting your paper, including quotations and citations, and developing your works cited page.

Turabian Style is a variation of Chicago frequently used in business and political science courses. 

A Turabian Quick Guide, with sample citations for different types of sources, is available on the University of Chicago Press website.