Access to inclusive school settings is associated with many benefits for students with intellectual and developmental disabilities in the areas of academics (Agran et al., 2020; Kurth & Mastergeorge, 2010), social skills (Kleinert et al., 2015; Lyons et al., 2011; Schwab et al., 2015), and communication skills (Ballard & Dymond, 2017; Kleinert et al., 2015), among others. Challenging behavior is a major barrier to inclusive school settings for some students with intellectual and developmental disabilities (Agran et al., 2020; Gee et al., 2020; Giangreco, 2020; Kleinert, 2020). Given the benefits of access to inclusive school settings, researchers and other educational team members must address this barrier. One way in which challenging behavior can be addressed in school settings, including inclusive school settings, is through function-based interventions. Function-based interventions are targeted interventions based on functional behavior assessment results and aligned with the function of a student's challenging behavior (Gage et al., 2012; Jeong & Copeland, 2020). It is critical that educators and researchers consider the contextual fit of a function-based intervention, as interventions that incorporate contextual fit are more effective than those that do not (Monzalve & Horner, 2021). Functional communication training (FCT) is an example of a highly effective function-based intervention implemented to teach functionally equivalent, socially appropriate communication skills and address challenging behavior. The literature on the effectiveness of FCT spans age groups, settings, interventionists, and disability categories (Cooper et al., 2020; Hume et al., 2021), including students with intellectual and developmental disabilities (Andzik et al. 2016; Walker, Lyon, et al., 2018). Although emerging evidence demonstrates the effectiveness of FCT implemented with students with intellectual and developmental disabilities in inclusive school settings (Masud et al., 2022), the research is minimal. Further, no FCT studies focus on a peer as an interventionist. The current study extends the literature on FCT for students with intellectual and developmental disabilities by demonstrating implementation by a peer mentor in an inclusive high school setting. The purpose of this study was to examine the effects of peer-delivered FCT on the functionally equivalent communication responses and challenging behavior of a high school student with intellectual and developmental disabilities, the collateral effects of the intervention on various student communication behaviors, and the degree to which educator team member participants and student participant found the intervention to be socially valid. Results of this study indicated that the FCT intervention had no effect on student functionally equivalent communication responses or challenging behavior. There is evidence of positive collateral effects as the student participant's rates of various communication behaviors increased after the intervention was implemented. Finally, all participants found the intervention to be socially valid across most measures, though there were concerns about the limited time and potential student discomfort. The dissertation includes a discussion of each research question, study limitations, directions for future research, and implications for practice. [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.]