While teaching in New York City public schools, the author began planning collaboratively with a team of mathematics teachers. Collaborative planning enabled them to consider a range of issues that may not have emerged had they been planning independently. In this article, the author first describes their work through excerpts of conversations that address four areas critical to successful collaboration: facilitation, sharing experiences, discussing mathematical concepts and teaching strategies, and sharing the planning workload. She shows how collaboration deepened their mathematical knowledge, supported dialogue on struggles with student ownership of content, and created a process of reflection for forward planning in reteaching units. (Contains 3 figures.)