"Technology-Enhanced Formative Assessment" (TEFA) is an innovative pedagogy for teaching secondary school science or mathematics with "classroom response system" technology. "Teacher Learning of TEFA" (TLT) is a five year research project studying teacher change in the context of an intensive, sustained, on-site professional development (PD) program designed to help in-service teachers learn TEFA. Approximately 39 teachers from three school systems are participating. Participants each receive a classroom response system and take part in a four-day summer workshop, a year of weekly PD meetings at their school, and one or two additional years of action research meetings every three to four weeks. Beginning with one baseline semester before the PD begins and continuing until the end of PD, we collect longitudinal data on participants' perspectives and practice through multiple instruments and methods, including interviews, web-based questionnaires, classroom videotaping, audio-taping of PD meetings, and surveys of participants' students. The data are analyzed and triangulated through a variety of qualitative and mixed-methods approaches in order to construct rich case study narratives of each participant's learning trajectory, and then to perform cross-case analysis in search of common patterns, distinct categories, and the like. Although the intervention, data collection, and analysis are ongoing, we have identified some preliminary findings. Based on these and detailed examination of a few initial case studies, we have developed a tentative model of teacher learning and pedagogical change in the context of sustained PD, which we call "a model for the co-evolution of teacher and pedagogy." The preliminary findings and model have implications for the design of professional development programs for in-service teachers. (Contains 3 figures.)