This article explores the relationship between teachers' mathematical knowledge and student progress in secondary mathematics. It presents findings from a 1-year study in Germany involving a representative sample of Grade 10 classes and their mathematics teachers. The study distinguishes between content knowledge (CK) and pedagogical content knowledge (PCK), finding that PCK has a substantial positive effect on student learning gains, mediated by cognitive activation and individual learning support. The research highlights the importance of both CK and PCK for high-quality instruction and student progress. It also discusses prior research on CK and PCK, emphasizing the need for a conceptual understanding of mathematical content and the role of PCK in instruction. The study concludes that PCK is a critical component of effective teaching, and that teachers' professional knowledge, including CK and PCK, is essential for providing insightful instruction and student learning. The study also examines the differential implications of CK and PCK for teaching and student progress, and the role of teacher training in shaping these knowledge components. The findings suggest that PCK is a major prerequisite for instruction that is both cognitively activating and adaptive, and that CK is necessary but not identical to a rich repertoire of teaching skills. The study also explores the interaction between PCK and secondary track, and the potential moderating effect of socioeconomic status on teacher effectiveness. Overall, the study underscores the importance of teacher knowledge in shaping instructional quality and student learning outcomes.This article explores the relationship between teachers' mathematical knowledge and student progress in secondary mathematics. It presents findings from a 1-year study in Germany involving a representative sample of Grade 10 classes and their mathematics teachers. The study distinguishes between content knowledge (CK) and pedagogical content knowledge (PCK), finding that PCK has a substantial positive effect on student learning gains, mediated by cognitive activation and individual learning support. The research highlights the importance of both CK and PCK for high-quality instruction and student progress. It also discusses prior research on CK and PCK, emphasizing the need for a conceptual understanding of mathematical content and the role of PCK in instruction. The study concludes that PCK is a critical component of effective teaching, and that teachers' professional knowledge, including CK and PCK, is essential for providing insightful instruction and student learning. The study also examines the differential implications of CK and PCK for teaching and student progress, and the role of teacher training in shaping these knowledge components. The findings suggest that PCK is a major prerequisite for instruction that is both cognitively activating and adaptive, and that CK is necessary but not identical to a rich repertoire of teaching skills. The study also explores the interaction between PCK and secondary track, and the potential moderating effect of socioeconomic status on teacher effectiveness. Overall, the study underscores the importance of teacher knowledge in shaping instructional quality and student learning outcomes.