To investigate teachers' everyday on-the-job learning, I used a comparative case study design and examined the work of mathematics teachers in 2 high schools. Analysis of interviews, classroom observations, and teachers' conversations highlighted 3 key resources for learning: (a) reform artifacts oriented the teachers' attention to key concepts of a reform, whereas the interactions surrounding them established local meanings; (b) conversation-based classification systems communicated pedagogical assumptions; and (c) the rendering of classroom interactions in conversations shaped opportunities for teachers to consult with and learn from colleagues. Taken together, these learning resources provide a conceptual infrastructure for teachers to make sense of their practice. This research highlights the social and situated nature of teachers' pedagogical reasoning and specifies the role of teacher community in teacher learning.