Much of the research on the development of professional noticing expertise has focused on prospective teachers. We contend that we must investigate practicing teachers as well, and in particular practicing secondary teachers, because they bring with them years of teaching experience and are situated in unique contexts. Hence we studied the longitudinal growth of the professional-noticing expertise of a group of practicing secondary teachers (N=10) as they progressed through a 5-year professional development (PD) about being responsive to students' mathematical thinking. Results indicated that the first half of the PD supported their interpreting and deciding-how-to-respond skills, and the second half of the PD supported their attending skills, which were already strong even before the PD. We compare these results with the activities that occurred in the PD and discuss implications for future research and PD programs. [For the complete proceedings, see ED629884.]