Risk importance measures are developed to quantify the worths of design features and human actions in both controlling and reducing risk. The quantification of worths can be used to help focus and prioritize efforts in backfitting programs, reliability assurance programs, inspection programs, and general risk management programs. The risk importance measures described are straightforward mathematically and have a natural physical interpretation. As a demonstration of their utility, the measures are applied to the probabilistic risk analyses (PRA) performed in the U.S. Nuclear Regulatory Commission Reactor Safety Study Methodology Applications Program. The results greatly enhance the information provided by the PRA and show extremely interesting behaviors. Within a plant and across plants, worths vary by orders of magnitude. Systems, components, and human actions that are important in reducing risk are not necessarily those that are important in controlling or assuring risk. The worths are graphically portrayed to more effectively communicate the messages to managers and decisionmakers. The applications indicate that the described importance evaluations should be an intimate part of any PRA.