This book tries to explain in detail, from a theoretical and practical perspective, the principles, structure, use and perspectives of the Universal Decimal Classification (CDU) in the context of documentary languages. It is not a work of divulgative character, but a specialized manual that analyzes with depth and rigor the foundations and possibilities of the cdu as documentary classification language of the libraries. Therefore, the text is preferably addressed to a specialized audience: students of librarianship and documentation, opponents and professionals in this area of ​​knowledge who want to update their conception and management of the Universal Decimal Classification.