Peer-mediated interventions are a powerful and practical way of promoting the social relationships, learning, and inclusion of students with disabilities. In this article, we describe one state's efforts to scale up a research-based, peer-mediated program called Peer to Peer throughout Michigan. Among the more than 700 schools that now offer this program, as many as 18,000 peers are involved in supporting nearly 5,000 schoolmates with autism and other developmental disabilities in their learning and relationships. We share our perspectives on eight key factors that have contributed to the growth and widespread adoption of Peer to Peer over the last 20 years. We discuss enduring challenges in this long-haul work and conclude with recommendations for future research focused on schoolwide peer-mediated programs.