This article investigates the significance of teachers' content knowledge (CK) and pedagogical content knowledge (PCK) for high-quality instruction and student progress in secondary-level mathematics. The study, conducted in Germany over one year with a representative sample of Grade 10 classes and their mathematics teachers, found that PCK was distinguishable from CK and had a substantial positive effect on students' learning gains, mediated by cognitive activation and individual learning support. The research highlights the importance of both CK and PCK in providing high-quality instruction and suggests that teacher training programs should focus on developing these two types of knowledge to enhance student learning outcomes.This article investigates the significance of teachers' content knowledge (CK) and pedagogical content knowledge (PCK) for high-quality instruction and student progress in secondary-level mathematics. The study, conducted in Germany over one year with a representative sample of Grade 10 classes and their mathematics teachers, found that PCK was distinguishable from CK and had a substantial positive effect on students' learning gains, mediated by cognitive activation and individual learning support. The research highlights the importance of both CK and PCK in providing high-quality instruction and suggests that teacher training programs should focus on developing these two types of knowledge to enhance student learning outcomes.