[en] The state of knowledge concerning postulated accidents involving offsite hazardous materials in the vicinity of nuclear power plants is critically evaluated. This effort is part of a study to analyze the potential effects of offsite hazards upon the safety of nuclear power plants and to develop a technical basis for the assessment of siting approaches. The evaluation includes consideration of data bases and statistics of hazardous materials and accidents involving them, deterministic aspects of possible material dispersion and threat environments, the susceptibility and vulnerability of vital plant systems, and a critical review of past licensing experience and regulatory practice with respect to these hazards. While many of the data bases and analysis methods exist for an adequate estimate of threat and plant response, this knowledge is not fully used and no comprehensive guidance has been developed. Siting of nuclear power plants relative to offsite hazardous materials is a risk based procedure that considers both probabilities and consequences of events that make up accident scenarios. In this context it appears feasible to improve the procedures vis-a-vis the perception of safety, economy of effort, and efficiency of implementation. A scenario dependent conditional risk approach is outlined as a possible means of improving the siting procedures