This study investigated relationships between changes in certain types of coaching knowledge and practices among mathematics classroom coaches and how these explain changes in the attitudes, knowledge, and practice of the teachers they coach. Participants in this study were 51 school-based mathematics classroom coaches in the USA and 180 of the teachers whom they coached between 2009 and 2014. The participating coaches were recruited from schools that hired their own coaches independently from this research project. This study found evidence that improvements in coaches’ use of practices recommended by particular coaching models are related to improvements in teachers’ mathematical knowledge for teaching. The study also found that improvements in coaches’ self-assessment of their own coaching skills are related to improvements in teachers’ mathematics content knowledge for teaching, mathematics teaching practices, and attitudes about self-efficacy for teaching mathematics. The study did not detect relationships between changes in coaches’ mathematics knowledge and changes in teachers’ knowledge or practices.