The Universal Decimal Classification (UDC) is a bibliographic and library classification representing the systematic arrangement of all branches of human knowledge organized as a coherent system in which knowledge fields are related and inter-linked.[1][2][3][4] The UDC is an analytico-synthetic and faceted classification system featuring detailed vocabulary and syntax that enables powerful content indexing and information retrieval in large collections.[5][6] Since 1991, the UDC has been owned and managed by the UDC Consortium,[7] a non-profit international association of publishers with headquarters in The Hague (Netherlands).

Unlike other library classification schemes that have started their life as national systems, the UDC was conceived and maintained as an international scheme. Its translation in world languages started at the beginning of the 20th century and has since been published in various printed editions in over 40 languages.[8][9] UDC Summary, an abridged Web version of the scheme, is available in over 50 languages.[10] The classification has been modified and extended over the years to cope with increasing output in all areas of human knowledge, and is still under continuous review to take account of new developments.[11][12]

Albeit originally designed as an indexing and retrieval system, due to its logical structure and scalability, UDC has become one of the most widely used knowledge organization systems in libraries, where it is used for either shelf arrangement, content indexing or both.[13] UDC codes can describe any type of document or object to any desired level of detail. These can include textual documents and other media such as films, video and sound recordings, illustrations, maps as well as realia such as museum objects.