The book of the head of the Federal Archival Service of Russia, Corresponding Member of the Russian Academy of Sciences V.P. Kozlov, continues a series of his studies on forgery of written sources on the history of Russia. The beginning of the series was laid by the author's works on falsifications of the 18th-19th centuries. This book tells about forgeries of the 20th century. In a special theoretical and methodological chapter, for the first time, a detailed typology of forgeries is given and the rules for their detection are formulated.