This three-year project targets elementary school teachers (grades 3-6) in the greater Boston area. The project's intent is to enhance teachers' understanding of central and pervasive mathematics concepts, and the processes by which mathematics is learned. The mathematics topics to be addressed are measurement, number systems, number relation ships, estimation, patterns and functions, and geometry. The project will emphasize the assessment of mathematical ideas, and the use of activity-based instruction. Two cohorts of teachers will be involved in summer work shops and sustained academic year activities. Leadership training will be provided in the third summer, and the teacher leaders will be involved in in-service work sponsored by the Massachusetts State Department of Education. Cost sharing is provided by the Massachusetts State Department of Education, the Education Collaborative for Greater Boston, and the participating schools as 19% of the NSF award.