Peer-mediated interventions (PMIs) offer substantial benefits to middle school, high school, and post-secondary students with intellectual and developmental disabilities (IDD). However, with no valid and reliable measure to capture the ways in which the peers without disabilities are impacted, far less is known about the outcomes for these students. These three interconnected studies detail the development and validation of a new measurement tool, the Peer-Mediated Impact Survey for Peers (PMIS:P). In Study 1, I conduct a phenomenological study (Moustakas, 1994) using a focus group methodology to understand the breath and range of ways peers are impacted by their involvement in PMI. In Study 2, I develop and pilot a new, content-valid measurement tool, the PMIS:P. In Study 3, I use the findings of Study 2 to refine the PMIS:P. After establishing the factor structure of the tool and assessing its reliability, I administered the final version of the PMIS:P to a large sample of peers. Findings from all three studies suggest that peers are impacted in varied and meaningful ways. Results of the factor analysis suggest there are seven categorical ways in which peers are impacted by their experiences: skill and intrapersonal development, self-worth impact, changes in views, social impact, advocacy impact, rewarding impact, and future impact. I discuss the validity and reliability of the measure. Additionally, I explore variables for which mean PMIS:P ratings significantly differed as well as variables for which ratings were not statistically significant. This new tool allows teachers, researchers, PMI program coordinators, and inclusive higher education program directors to better understand how peers are impacted by their involvement in PMI. I suggest future directions for research and address 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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]