National assessments and other investigations have drawn attention to the fact that there is considerable room for improvement in science teaching and learning. This article describes, exemplifies, and discusses a research program set up to address this unsatisfactory state of affairs. Briefly, the idea of the program is that researchers in science education and teachers in schools should work together to design teaching sequences and assess how they function in practice, including what the students learn. This work has two aims: first, to generate results of practical use, and second, to contribute to the development of educational science. Results of practical use are presented in the form of teacher's guides, written to be used as tools for generating further knowledge. As a contribution to educational science, attempts are made to formulate content-oriented theories of conditions that are likely to improve learning with understanding. This article describes and discusses how the research program has been implemented in the area of geometrical optics at the upper level (grades 8 and 9) of compulsory school. It is suggested that the idea of content-oriented theories is worth further examination and development and might contribute to strengthening science education as an autonomous discipline.