This study examined the effectiveness and impact of collaborative coaching, a special type of professional development, on the views and actions of a secondary school mathematics teacher. In particular it examined the following two research questions: 1) In what way did collaborative coaching that specifically focused on the MKT of a secondary school mathematics teacher affect her views on the teaching and learning of mathematics?, and 2) In what way did the participant attend to students mathematical contributions differently because of her participation in the collaborative coaching? In designing this study, the researcher reviewed literature on teacher knowledge, professional development, and teacher learning. The collaborative coaching focused on the mathematical knowledge for teaching (MKT) of the participant and the researcher. Coaching sessions comprised of both participant and researcher watching video clips from the participants two Gifted/Honors Algebra II classrooms and working on the mathematics contained in the video clips. The video clips mostly contained students mathematical contributions from class and the aim of most coaching sessions was to improve, elaborate, discuss, or expand on these mathematical contributions and hence, as a pair, enhance the MKT of both participant and researcher. The participant changed her views on teaching as a teacher-driven endeavor to an exercise with more student involvement. She changed her views on learning as something that was done by passive students taking notes and practicing problems to students actively engaging in mathematical discussions and arguments and learning through this engagement. She changed her attention to students mathematical contributions by focusing on these contributions for longer periods of time and facilitating interactive student engagement with the contributions rather than merely validating or confirming the contribution herself.