Analyses conducted by the Nuclear Regulatory Commission (NRC) indicate that timely and effective protective action would be necessary to protect the public in a major nuclear power plant accident. Given the large amount of time required to implement an evacuation around most reactor sites, protective action recommendations (PARs) must be based upon specific plant indicators regarding the status of the core and systems that protect the core. This article describes the assumptions made, and the analyses conducted, by the NRC in developing its procedures for PARs based upon plant conditions.