This study examines the role of several key features of professional development (PD) in bringing about changes in teachers' mathematical knowledge for teaching. Artifacts (e.g., PD tasks, materials, and agendas) of the PD activities completed by 542 teachers in 21 different Mathematics and Science Partnership programs were coded based on the core features of PD identified in earlier literature. Multilevel analyses examined whether the programs' focus on specific components of teacher knowledge (e.g., knowledge of mathematics teaching) and specific strategies for implementing the PD content (e.g., examining students' work and solving mathematics problems) were related to gains in teachers' mathematical knowledge. The results showed that a focus on curricular content knowledge and examining students' work were significantly related to teachers' learning. Implications for research and teacher education are discussed.