This paper discusses the theoretical scope and practical applicability of generalizability (G) theory through the principle of symmetry. Major ideas are summarized and factors hindering applications of G theory in research conducted in French-speaking Europe are presented. The principle of symmetry affirms that any factor of a design can be selected as an object of measurement and that the G theory operations defined for one factor can be transposed in the study of other factors. The principle allows the extension of G theory to situations based on complex factorial designs and involves multiple purposes of measurement in three major directions: (1) consideration of all types of facets; (2) analysis of multifaceted populations; and (3) development of a general framework for analysis. Widespread application of G theory is unlikely to occur until specialists in program evaluation develop procedures for integrating the collection and analysis of quantitative data with the application of quantitative methods of investigation. One of the most potentially useful applications of generalizability theory is the procedures it provides for using data from an initial study to determine improvements of the design to be used in subsequent research or in decision making. (PN)