This report describes the rationale, development, technical characteristics, and use of the ACT Interest Inventory, which provides both descriptive information about an individual's interests and information to facilitate focused exploration of educational and career alternatives. The report begins with consideration of the question, "What are interests and why do we measure them?" It concludes with a discussion of the ways information about group differences in educational and vocational behavior can be used to help individuals make satisfying and rewarding career choices. The results of extensive item analyses indicated that the scales of the ACT Interest Inventory measure six basic dimensions of interest with sufficient consistency and accuracy for individual counseling with students. Group differences in item responses suggested that separate scaling and norming procedures were needed for men and women, but that whites and non whites did not require separate norms. Validity evidence supporting the descriptive use of the ACT Interest Inventory included correlational information showing that the six scales are relatively independent, are interrelated in the expected circular fashion, are highly related to Strong Vocational Interest Blank scales measuring the same dimensions, and are unrelated to ACT ability measures. (Author)