UDC is used in bibliographic services, documentation centres and libraries in around 130 countries world-wide. For an indexing systems used internationally for over 100 years it is difficult to provide accurate account. Most of information on this exist in national statistics and bibliographic and library centres in indivdiual countries. In the past there were many surveys conducted nationally and internationally on the UDC use. Some overviews based on more recent research data are provided in separate pages.