The Chicago Manual of Style (CMOS) is defined by the University of Chicago Press and consists of two different systems:

Notes and Bibliography

Sources are cited in numbered footnotes or endnotes. Each note corresponds to a raised (superscript) number in the text. Sources are also usually listed in a separate bibliography.

This system is preferred by many scholars in the humanities, including history and the arts.

Author-Date

Sources are briefly cited in the text, usually in parentheses, by author’s last name and year of publication. Each in-text citation matches up with an entry in a reference list, where full bibliographic information is provided.

This system bears similarities to MLA style and is preferred by many scholars in the sciences and social sciences.

• Notes and Bibliography: Sample Citations (directly from the CMOS)
• Author-Date: Sample Citations (directly from the CMOS)

These links use the most recent edition of CMOS, the 18th edition (2024).

Purdue OWL's guide is currently based on the 17th edition (2017).

For a quick list of differences between the 17th and 18th editions, see "What’s New in the 18th Edition." Some of the most notable changes are:

• New guidance on citing AI-generated content (images and text);
• Place of publication is no longer required when citing books;
• A page range  is no longer required when citing a chapter in an edited book.