Research on professional development (PD) typically focuses on what teachers learn as a result of their participation in PD. Questions are framed unidirectionally: To what extent does participation in PD affect teachers' classroom practice? The authors challenge this unidirectional conceptualization of teacher learning and instead argue for understanding the multidirectional influences between teachers' participation across the PD and classroom settings. Drawing on research in mathematics education, they argue that researchers should examine what teachers are learning during and after PD, looking at the "coevolution" of participation between classroom practice and PD. To advance studies of and designs for PD, the authors' perspective leads them to argue for the importance of better understanding how teachers come to make sense of primary artifacts, depictions, and enactments in and through PD. (Contains 6 notes and 1 table.)