Walton, MargaretFacilitators of teacher professional development (PD) play an integral role in teacher learning. Facilitators often both plan and implement PD and it is important that they can make these experiences meaningful learning opportunities for teachers. Researchers have only recently delved more deeply into understanding the knowledge and skills facilitators need for their work, and how to support facilitators to learn such knowledge and skills. This qualitative three-article dissertation is a design-based research project that explores what facilitators do and how they learn to support teachers in developing a particular instructional skill- noticing student mathematical thinking. Noticing student thinking is how teachers center and build on student ideas in the classroom. I designed a facilitator PD (F-PD) that aimed to help six novice facilitators learn to lead video clubs, a type of teacher PD that has been shown to support teachers in learning to notice. I examined how the novice facilitators learned to lead video clubs and how characteristics of F-PD supported or constrained that learning. In the first study, “A Facilitator Noticing Framework: How Facilitators Notice Teacher Thinking,” I develop a framework for facilitators’ cognitive process as they support teachers to learn to notice in PD, like video clubs. I argue that, like teachers, facilitators also notice. However, facilitators primarily notice teacher, rather than student thinking. I explain the different aspects of teacher thinking that a facilitator might notice. I then use the framework as a lens to understand how three experienced facilitators’ interactions with teachers in video clubs support the teachers to notice student thinking. Study Two, “Novice Facilitators Learning to Lead Video Clubs: A Framing Perspective” is a close examination of how the participants in my F-PD learned to lead video clubs. The analysis included qualitative coding of the participants’ focus related to leading video clubs during discussions with each other and me as the F-PD leader. The findings indicate that participants’ understanding likely shifted. Early in the F-PD, participants appeared to think of leading video clubs as sustaining any general conversation between teachers. Later in the F-PD, the participants likely understood video club facilitation as paying attention and responding to aspects of teachers’ thinking related to noticing student thinking. The interactions between the participants and me, along with the F-PD design, appeared to contribute to this shift, which is also explained. In Study Three, “Designing to Support Facilitators to Learn to Notice Teacher Thinking,” I zoom out and look at the F-PD as an overall activity. I identify some of the problems that arose during the F-PD that constrained participants' learning. I explore how I changed the F-PD design in response or, how differences in the F-PD design from early to later session mitigated issues. I offer several design suggestions for future F-PDs, based on my findings.enDissertationMathematics educationTeacher educationFacilitator NoticingProfessional DevelopmentTeacher NoticingVideo Clubs