This document, the result of the Youth in Transition longitudinal study, describes procedures, rationale, and major results of an evaluation of American high schools. The purpose of the study was to examine factors that make some schools superior to others. A "good" school is defined as one which has a greater than average desirable impact on its students and a lower than average undesirable one. Over 1300 young men from 87 high schools provided data for the study by answering questionnaires. The approach was to consider all school characteristics that might make a difference for students. The data, consolidated and reduced from some 1600 to 32 variables, were categorized under headings of knowledge and achievement, behaviors, plans, attitudes and values, affective states, motives, aptitudes, abilities, and school reputation. Separate chapters of this document describe the present state of knowledge in the field, the research design, concept and instrument development, major data collections, and descriptive information on American high schools. Findings indicate that high schools have virtually no differential impact on student development. The author suggests a major social experiment in which diverse forms of education are created. Suggestions to change the organizational structures of formalized schooling are also presented. Appendices include ten questionnaires and student interviews. (KC)