The article contains a review of underlying concepts and definitions of non-observed economy which are formulated in the international standards on this topic and used by the CIS countries for computation of GDP and other key indicators of the System of National Accounts. The article considers the methods used for measuring non-observed economy and some figures on the share of non-observed economy in GDP of the CIS countries and other selected countries of the world. Perfection of methods of measuring non-observed economy and raising reliability of its estimates requires improvement of general level of work on compilation of national accounts.