Most supervisors lack training in best practices in clinical supervision (Newman et al., 2019). However, supervisees report more positive supervision experiences from supervisors who have received training (Brott et al., 2016). To address this need, a group of university educators and an experienced field supervisor collaborated with the Illinois School Psychologists Association (ISPA) to develop the ISPA Supervisor Credential Program, a no-cost, 14-hour supervision course based on the clinical supervision principles of the developmental, ecological, problem-solving (DEP) supervision model (Simon & Swerdlik, 2023). This article summarizes findings from focus groups and surveys conducted as part of a formal evaluation of the ISPA Supervisor Credential Program. It summarizes implications for best practices in professional development for supervision, highlights supervisory practices that supervisors find most effective, and provides recommendations for individual and system-wide professional development in supervision competencies.