Action research shifts the paradigm of contemporary educational reform by emphasizing inquiry and placing teachers at the center of research-into-practice. By situating teachers as learners, action research offers a systematic and intentional approach to changing teaching. When working as part of a community of practice, action researchers engage in sustained professional learning activities. They explore issues of everyday practice and work to bring about change. This review highlights action research studies from across four subject areas--English language arts, mathematics, science, and social studies--and is premised on the notion that changing teaching practice is connected to understanding how teachers learn. Specifically, it focuses on understanding changes in teacher pedagogical content knowledge, disciplinary inquiry, and critical pedagogy through action research. Findings suggest that we must go beyond current conceptualizations of teacher learning as process-product, cognitive, and situative to view teaching as inquiry. Successful efforts to change practice through action research have demonstrated the value of engaging teachers as active participants in education research. At the same time, the field must overcome barriers including the marginalization of action research, logistical issues associated with conducting action research, and the dissemination of findings.