Graduate teaching assistants (TAs) are an important group of math instructors whose practice deserves to be supported and studied. In this action research study, I lead a customized regimen of professional development (GQuesT model) for a cohort of first/second-year calculus TAs and study their teaching practice. This study focuses on how the TAs prepare for, orchestrate, and reflect upon class-wide mathematical discussions as they set mathematical goals, pose purposeful questions, and use evidence of student thinking. The results include a detailed, collective case study of the five TAs participating in professional development and leading lab activities. This study suggests that collaborative professional development focused on developing important teaching practices, centered around common lab activities has the potential to support rich discussion among TAs who are looking to improve their practice. This study highlights the usefulness of attending to how TAs collect evidence about whether their mathematical goals are achieved in the classroom, as this impacts how they interact with their students' ideas. Recommendations for future professional development efforts with math TAs include planning for their proclivity towards mathematical correctness and sorting appropriate activities for TAs at varying levels of teaching maturity. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]