This communication introduces a comprehensive theory for process security, assessment. The theory can quickly provide mathematically proven upper and lower bounds on minimum time to disaster (the time disaster conditions will be reached in worst-case scenarios). Computational efficiency and reliability are achieved by performing process security assessment based on a time-derivative-based approach, rather than conducting massive process dynamic simulations. Further, the methodology enables prioritizing the process variables critical to plant security. This facilitates pinpointing the possible reasons of a security-threatening situation, quantifying the importance of each disturbance factor on the system's vulnerability, and creating a prioritized list of improvements/ counteractions for ensuring process security. Two runaway reactor examples are studied to demonstrate the methodology for security-threat analysis. (C) 2004 American Institute of Chemical Engineers.