We report on a teacher development program aimed at improving mathematics teaching and learning from grades 5-9. The 18-month largely online program arose from a partnership of mathematics education researchers, mathematicians, physicists, and nine school districts. The program employed functions as a lens to reinterpret and interconnect content topics in the mathematics curriculum. It was designed to promote a deep understanding of mathematics while drawing upon students' spontaneous reasoning, generalizations, representations, and discussions about relations between physical quantities and numbers. We describe the program's foundations and activities and analyze changes in teaching and in student learning during 3 years of implementation. Results suggest that the teacher development program contributed to improved teaching in the target districts and to positive changes in the performance of target district students on state-mandated tests.