The identification and assessment of threats provides an essential basis for the selection, design, and implementation of nuclear security measures. For nuclear material and other radioactive material that is under regulatory control, and associated facilities and activities, the results of this identification and assessment are expressed as a design basis threat or representative threat statement describing the intentions and capabilities of potential adversaries against which the materials and associated facilities and activities are to be protected. An Implementing Guide was issued in 2009 under the title of Development, Use and Maintenance of the Design Basis Threat which was updated and revised. The result of this revision is the current publication. It provides a step-by-step methodology for conducting a national nuclear security threat assessment including both physical and computer security aspects, and for the development, use and maintenance of design basis threats and representative threat statements.