This study examines elementary- and middle-grade teachers' understanding of the mathematical learning progression as they participated in a 6-month professional learning project. Teachers participated in a professional development project that consisted of a 1-week summer content-focused institute with school-based follow-up Lesson Study cycles in the fall that focused on the vertical articulation of algebraic concepts across grade levels. The following research examines how a vertical team of teachers from multiple grades designed, taught and learned from the Lesson Study cycle. The video analysis from the research lessons and the teachers' reflections revealed teachers' developing vertical connections and representational fluency from their planning, teaching, observation and debriefs. In addition, Lesson Study afforded teachers opportunities to deepen their understanding of the mathematical learning progression through observation and analysis of students' thinking through a situated school-based professional development experience.