This report is a response to what is perceived as the need for a systematic collection of information on the effects of semestering a secondary school, particularly as those effects are felt in teaching and learning. Chapters deal with monitoring the present school organization in the areas of student course selection and credit accumulation; the new patterns in school organization, including full-credit semestering, half-credit semestering, trimestering, mixed models, variable period-length plans, special timetable cycles, summer schools, and extended-year plans; an explanation of full-credit semestering, including what appear to be its three main advantages--more effective teaching/learning experiences, more efficient utilization of the school year through the provision of more entry and reentry points and greater opportunities for students to make up courses or accelerate their programs, and more effective and efficient utilization of materials, facilities, and other resources; and the implementation of organizational change. (Author/IRT)