The Dewey Decimal Classification system, sometimes abbreviated DDC, is a method of categorizing books in a library by subject matter. It is a numerical system using groupings of ten — i.e. there are ten major classes, each of which has ten divisions, each of which has ten sections — and books are placed on the shelf in numerical order. The Dewey Decimal system was created by Melvil Dewey in 1876 and is currently owned by the Online Computer Library Center in Dublin, Ohio, who acquired the trademark in 1988.

The Dewey Decimal system is extensively used in United States libraries, and a system based on it, the Universal Decimal Classification, is used around the world. Before the Dewey Decimal system was developed, there was no standard of organizing library books, and most systems in use were quite arbitrary and inefficient. Today, the Dewey Decimal system is used in about 95% of United States school and public libraries, while the Library of Congress Classification system, first developed in 1897, is more widely used in government and university libraries.

The call numbers in the Dewey Decimal system provide increasingly specific information about the book when read from left to right. There are three numerals in each, followed by a decimal point, which may be followed by more numerals to more specifically categorize the book. The second line of the call number consists of the first few letters of the author's name, which may be used to alphabetize books with the same numerical classification.

Though fictional works are classified in the Dewey Decimal system within the 800 class, Literature, many libraries choose to have a separate section for fiction in which the books are alphabetized by author. In the Dewey Decimal system, literary works are subdivided by their original language and then by form — poetry, fiction, or essay, for example — so that all works of fictional prose are not in a single place. Making a separate section for fiction both caters to patrons who are only interested in novels and keeps the 800 section from becoming overgrown.

The ten main classes of the Dewey Decimal system are as follows:

000 – Computer science, information, and general works
100 – Philosophy and psychology
200 – Religion
300 – Social sciences
400 – Language
500 – Science
600 – Technology
700 – Arts and recreation
800 – Literature
900 – History and geography