The author makes several prefatory observations on the hodgepodge nature of current social studies programs, and the lack of a functional (as opposed to descriptive) statement of the means and ends of social studies education. The following functional definition, developed by the staff of the Marin Social Studies Project, is offered: The social studies is that portion of the general education curriculum the purpose of which is to make students more rational with regard to human behavior and social interaction. In dissecting the individual parts of this statement, the author interprets the criterion of rationality and its value as a guide for content selection; explains the inclusion of the terms "social interaction" and "general education", and points up the utility of this definition for teachers and curriculum designers in making decisions about what they do. A rationale for this definition, based on the needs of students, is presented, and the implications of the definition for the development of an improved K-12 curriculum are briefly dealt with. (JLB)