This handbook examines practices and issues involved in the implementation of multiage programs. Data were collected through site visits at four schools with reputations for excellent multiage programs; interviews with a representative sample of parents and teachers at each school; document analysis; a survey of multiage teachers, parents, and principals at each school; and a survey of participants attending a national conference on multiage instruction. Before implementing multiage programs, six important questions should be asked regarding: the reasons for implementation, teacher roles, type of school or organizational climate, type of parent and community involvement, leadership, and factors for success that can be generalized to other settings. The following guiding principles were identified: (1) There are compelling benefits for students and teachers that justify implementing multiage organization; (2) there is no single model; (3) neither bottom-up nor top-down implementation, by itself, is effective; (4) multiage programs require major conceptual change; (5) implementation is best viewed as an evolving, long-term change at the deepest levels of teacher beliefs about how humans learn; and (6) several incremental steps can facilitate and improve the likelihood of success. A total of 17 tables and 3 figures are included. Appendices contain information on the study methodology, data collection instruments, and codebook. (LMI)