This publication provides guidance on how to develop, use and maintain a design basis threat (DBT). It is intended for decision makers from organizations with roles and responsibilities for the development, use and maintenance of the DBT. This implementing guide describes a DBT; identifies and recommends the roles and responsibilities of organizations that should be involved in the development, use and maintenance of a DBT; describes how to conduct a national threat assessment as a precursor to a DBT; explains how a DBT can be developed; explains how a DBT is incorporated into a State’s nuclear security regime; and explains the conditions for a review of the DBT, and how the review and update are conducted.