The purpose of this study conducted by the Wisconsin Research and Development Center for Cognitive Learning was to develop, implement, and evaluate an experimental preschool education program based on Piaget's theory of cognitive development. A second purpose was to examine Piaget's theoretical assumptions and postulated cognitive developmental trends for a two-year period of the preoperational substage. A core group of 48 subjects aged 3 to 5 years remained with the project for the full two years. The document includes sections on (1) the preoperational characteristics of mental operations, (2) description of preschool programs and the teacher education component which includes principles, guidelines and components of the Piagetian preschool program, a conventional nursery school program and the teacher education program for the Piagetian Preschool Education Program (PPEP), (3) methods and procedures, (4) results and discussion. Extensive tables are included. The 92 pages of appendices include curriculum planning materials, task protocols, scoring procedures and materials as well as comprehensive score patterns and stage designations for the complete task battery. (MS)