This study is a qualitative analysis of data collected during a yearlong series of teacher learning community meetings and classroom observations. The participants are middle and high school mathematics teachers from 2 school districts. Teachers were introduced to the research behind formative assessment and how to apply that research to their teaching during a 3-day summer workshop. Teachers then met monthly in small school-based groups to deepen their understanding of formative assessment and to talk about their own classroom experiences. The analyses focused on how teachers' understanding changed over time, how the teacher learning communities supported the teachers, how learning translated into classroom practice, and the factors that supported or hindered the development of teachers' understanding and practice. Lessons learned during the study itself and from subsequent analyses of the data had a significant impact on the development of ETS's "Keeping Learning on Track"® program.