Why people join voluntary associations has been of interest to Americans since the publication of Alexis de Tocqueville's observations on the importance of such associations in American society. The search for explanations for joining behavior was first motivated by a desire to know and understand this active American phenomenon. In more recent years, the motive has become more pragmatic; associations are interested in understanding the phenomenon to control membership enrollments. This study is an extension of the work which has evolved from this more recent interest. A theory of affiliation with professional associations was developed and tested with state and regional samples of elementary principles. Discriminant analysis was applied to determine the effectiveness of the variables as a group and individually in distinguishing members from nonmembers. The best predictors, in order, at the state level were benefits of membership, the principal's attitude toward associations, membership at the national level, and the principal's social needs. The best predictors, in order, at the national level were benefits of membership, superior's attitude toward associations, membership at the state level, and the principal's attitude toward associations. / Ed. D.