Teachers' content knowledge and beliefs about teaching and learning are among the key factors for effective teaching and, in turn, for student achievement-related outcomes. This study explores the extent to which K-8 math teachers'--who teach in high-poverty urban schools--professional background, motivational beliefs, and mathematical knowledge for teaching (MKT) have an impact on students' math achievement. Hierarchical linear modeling (HLM) results indicated that although students' prior mathematics achievement was the most determining factor of their subsequent math achievement, teachers' MKT and holding a bachelor's degree in mathematics had significant positive effects on students' math achievement. Results provide support for professional development (PD) to focus on improving mathematics teachers' mathematical knowledge for teaching. Results may also have implications for education policies at both the district and state level for teacher incentives to further develop teachers' mathematical knowledge for teaching, especially for urban school teachers.