This report is Part I of a study of program implementation in 12 models of Head Start Planned Variation. Chapters examine (1) how well models are implemented, using sponsor ratings of teachers as the data source; (2) the factors which may influence the extent to which models are implemented, such as sponsor input, staff reaction and input, and the context in which implementation is undertaken; (3) the correlations between the factors which explain variations in levels of implementation; and (4) what the model classrooms are like in practice, using classroom observation data. Included are a summary of findings, recommendations for future implementation studies, and data tables. (SET)