The extended school year is based on the needs to economize, to improve teacher status, to deal with the teacher shortage, to improve and enrich education for all pupils, to achieve pupil acceleration, and to keep youth busy and off the streets during the summer months. In spite of these claimed advantages, experimentation and adoption of extended school year plans have not been widespread. After a discussion of the traditional summer school plan, this report summarizes the operations, experiments, and studies which have been conducted on each of the following plans: (1) Rotating four-quarter, (2) continuous four-quarter, (3) extended school year, (4) continuous school year or continuous progress, (5) multiple trails, (6) modified summer school, (7) trimester, (8) quadrimester, and (9) extended K-12. A bibliography containing 62 books and articles, 22 research reports, and five research summaries is appended. (DE)