School systems around the world are adopting more intellectually ambitious academic content in the hopes of improving their educational productivity. In the United States, these efforts have required significant changes to teachers' instructional practices, and increased attention to teachers' formal and on-the-job professional learning opportunities. Focusing on the initial implementation of a reform-oriented approach to teaching mathematics in two local school systems in the United States, this paper examines whether -- and under what circumstances -- elementary teachers' professional learning opportunities predict changes in their instructional practices and beliefs related to mathematics. Our findings reveal that teachers' on-the-job interactions during the first year of reform predicted changes in their instructional practices; further, these changes were associated with the infrastructures that local school systems created to support teacher learning in mathematics. Teachers' participation in formal professional development, in contrast, predicted changes in their instructional beliefs, but not their practices. These findings have implications for future efforts to design formal and on-the-job professional learning opportunities that support ambitious educational reforms.