Though system-building is a central focus of US educational reform, we know little about the relationships between school systems and instruction or its improvement. Though research on how school systems influence instruction has grown, most such studies attend to one or two policy initiatives, e.g., standards, assessment, or professional development. Researchers have learned a good deal from this work, but we have not learned very much about how system components work in interaction rather than in isolation: i.e., how they work as systems. That is what we propose to do: to study how the key components of school systems interact to influence instruction and instructional improvement. We will do so through a program of descriptive, theory building qualitative research investigating how essential components of school systems interact to influence instruction in six diverse systems.