This book describes the Missouri Mathematics Effectiveness Project, a major systematic research program supported by the National Institute for Education. The report is based on a decade of naturalistic and experimental study of mathematics classrooms in intermediate elementary grades and junior high school classrooms. A rationale for the particular research problems addressed, the research procedures and findings from several studies, and implications for teachers, teacher educators, and classroom researchers are each discussed. The focus is on the relationship between classroom processes (e.g., teacher behavior) and classroom products (e.g., student achievement). The ten chapters include the introduction/overview, a description of the naturalistic study, building a treatment program, reports of experimental work in the schools, a review of the literature on teaching and learning styles, a description of the Missouri Mathematics Program, the role of development in mathematics teaching, and conclusions and new directions. An appendix presents the instruments for assessing students' and teachers' beliefs about mathematics. (MNS)