Book excerpt: This revision provides guidance on how to establish or improve, develop, implement, maintain, and sustain computer security within nuclear facilities. This publication addresses the use of risk informed approaches to establish and enhance computer security policies, programmes; it describes the integration of computer security into the management system of a facility; establishes a systematic approach to identifying facility functions and appropriate computer security measures that protect sensitive digital assets and the facility from the consequence of cyber-attacks consistent with the threat assessment or design basis threat.