This study used knowledge derived from classroom-based research on teaching and laboratory-based research on children's learning to improve teachers' classroom instruction and students' achievement. Twenty first-grade teachers, assigned randomly to an experimental treatment, participated in a month-long workshop in which they studied findings on children's development of problem-solving skills in addition and subtraction. Other first-grade teachers (N=20) were assigned randomly to a control group. Although instructional practices were not prescribed, experimental teachers taught problem solving significantly more and number facts significantly less than control teachers. Experimental teachers encouraged students to use a variety of problem solving strategies, and they listened to processes their students used significantly more than did control teachers. They believed that instruction should build upon students' existing knowledge more than did control teachers, and they knew more about individual students' problem-solving processes. Experimental students' exceeded control students in number fact knowledge, problem solving, reported understanding, and reported confidence in problem solving. (Author)