It is very common for second language learners to encounter difficulties with a structure or concept, even after repeated explanation and practice.2 Yet foreign language teachers who wonder about the best way to address these structure or conceptual problems may find little help when they turn to their textbooks or current research on language peda gogy. Second language acquisition research is not necessarily concerned with solving classroom problems and, in the field of language pedagogy, there are frequently more questions than answers. One such question that we have posed recently is why second language learners experience the problems they do when it comes to the acquisition of German case morphology. Clearly, this question is not only of pedagogical interest for language teachers but also of importance for researchers who seek to better understand second language ac quisition. Thus, although teachers and researchers may approach a problem differently, their concerns are interrelated. Nevertheless, many scholars such as Ellis choose to focus on the gap that sometimes separates practitioners and theorists: Teachers require and seek to develop knowledge; researchers endeavor to advance technical knowl edge. This distinction ... encapsulates the divide that often exists between the two (39).3 In this arti cle, we adopt a more integrative view (see van Lier and O'Brien) which emphasizes the unity of prac tice, theory and curriculum. Like these scholars, we believe that attempting to maintain a strict dichot omy between practical and techni cal knowledge' (or as it is more often posited, and theory) is counterproductive, since improvement of practice and production of knowledge are not mutually exclusive goals. The study reported on here involved a collabo ration between teachers and researchers, with the goal of engaging together in a cyclical process of identifying, investigating, and trying to solve a specific pedagogical problem. This process of in quiry, commonly referred to as action research, was first envisioned by Lewin, and further devel oped by Kemmis and McTaggart, Gregory, Wallace and others. As we hope to demonstrate here, ac tion research can be a powerful tool to help teach ers understand their second language learners and discover modifications in teaching that may benefit them.