One of the challenges facing the designer of a standardized plant is related to the following question: “What are the required characteristics of a plant design that would permit that plant to be located on virtually any site in the United States?” The answer to that question is made difficult and complex due to a great number of site related factors. The consideration of these factors in a comprehensive manner comprises the elements of what we shall call a “siting envelope.” This definition goes beyond site considerations only, since it involves the acceptability of a particular plant design to be placed on a particular site. A number of independent studies are being made to define the range of values to be associated with specific site parameters in different regions of the United States. A number of these studies have been published, are established as Atomic Energy Commission Regulatory Guides, or are in the process of being developed as industry standards. Similarly, the methods of defining design acceptability are in the process of being established.