This qualitative case study describes how one beginning primary grade teacher benefited from collaborative lesson-planning meetings with her grade-level colleagues. The teacher accumulated knowledge of curriculum, pedagogy, and professional contexts as she participated in planning meetings each week during her first year of teaching. Furthermore, sustained, active participation in content-specific planning meetings changed the teacher's efficacy for mathematics instruction over time. Findings suggest that both participation and collaboration with colleagues during planning meetings were central to the teacher's knowledge development and sense of efficacy as a new teacher.