Since 1983, the Educational Technology Center at the Harvard Graduate School of Education has studied the uses of computers and other technologies to improve K-12 instruction in science, mathematics, and computing. Collaborative research groups--including scientists and mathematicians, practicing teachers, learning theorists, and software designers--have focused on "targets of difficulty," or curricular topics that are both crucial to students' further progress in these fields and widely recognized as difficult to teach and learn. More than 15 research projects have been completed to study the nature of students' difficulties, clarify the educational advantages that computer technology offers, and design experimental lessons that use computers as well as traditional materials to address these difficult topics. Three research groups tried promising teaching units in five Massachusetts high schools to learn about their use in regular classrooms and schools. This monograph summarizes the results of the Educational Technology Center's work and their implications for policy makers, school practitioners, and others concerned about science and mathematics education. The findings of the research project fall into three categories: (1) teaching and learning; (2) technology; and (3) implementation. (PK)