A research study was conducted by two teams of researchers who investigated how class size influences teaching and learning. Four second grade teachers participated, two from a rural school in Virginia and two from an urban, inner-city school in California. To understand how the same teachers and students responded to different class size conditions, a design was used allowing the researchers to change class size in the middle of the school year. Part I of this report on the research study includes a chapter on methodology, describing in detail the approach to the study. In part II, a description is given of the school year in Virginia, beginning with a description of the school setting, continuing with a chapter about each of the classes, and concluding with a discussion of what changes took place in instructional methods and student reactions after the class size was changed. Part III presents this same sequence of chapters for the school in California. Part IV presents a synthesis and discussion of results from the two schools. Details are provided about the collection and analysis of data as a comparison of data across sites. Part V offers a discussion on the implications for conducting research for teaching and learning. (JD)