While there is no shortage of evidence-based interventions designed to address the academic and behavioral difficulties of students in schools, it can be difficult to find adults that are available to implement them consistently. To remedy this problem, students' peers have often been utilized effectively as academic interventionists in the schools and as behavioral interventionists for students with disabilities. Two studies were designed to investigate the effectiveness of a peer-mediated intervention for students at-risk for developing behavioral disorders. Check-in/Check-out (CICO), a mentor-based intervention traditional implemented by adults, was modified so that it could be easily implemented by elementary school students. Using a reversal design, Study 1 examined the effectiveness of peer-mediated CICO utilizing elementary school students as interventionists for their behaviorally at-risk peers. Study 2 compared the relative effectiveness of adult and peer interventionists in a CICO framework using an alternating treatments design. [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.]