This action research study in a North Dakota public school addresses the need for effective professional development (PD) to enhance instruction and academic outcomes for mathematically promising learners. Despite annual PD investments, concerns persist regarding the limited efficacy of short-term workshops (Knight & Skrtic, 2021; Kraft et al., 2018). Instructional coaching offers promise in implementing research-based instructional methods (Johnson, 2016). However, empirical support for coaching in differentiated instruction for high ability learners remains scarce. This study examined coaching's impact on first-grade teachers' differentiation practices for mathematically promising learners. It was selected as despite teachers' awareness of differentiation benefits, many struggle with implementation, necessitating targeted support (Gavin, Casa, Firmender, and Carroll, 2013; VanTassel-Baska et al., 2020; Yuen et al., 2018). Through a mixed-methods approach, data were collected via teacher surveys and learner achievement growth scores to assess changes in differentiation practices, effects on learner performance, and influence on a teacher's sense of efficacy. Results indicated noticeable shifts in teacher practice and increased sense of efficacy, despite a lack of improvement in learner achievement outcomes. Coaching positively influenced teacher perceptions and confidence surrounding differentiation practices, but improving academic performance for high ability learners remains complex. This dissertation contributes to instructional coaching and differentiation literature, highlighting coaching's potential to support educators in serving mathematically promising learners and informing the development of more effective PD initiatives. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]