School counselors serve an important role in providing frontline counseling services to students in local school districts. Professional associations and educational institutions emphasize the importance of professional competence. However, research on professional competence is fragmented, outdated, and unintegrated. This qualitative study explored the development of competence in trainee school counselors during supervision, from the supervisors' perspectives. Using the conceptual frameworks of self-efficacy and professional identity development, 16 experienced supervisors were interviewed and asked to describe the development of competence in trainee school counselors and how they instructed the development of competence. Thorne's interpretive description method was combined with a reality-testing framework to guide the research procedures and data analysis plan. Following an inductive process, 663 codes were distilled into 37 themes, which resulted in 17 assertions. It was found that competence was gained across a continuum of growth and was recognized as either self-confidence or self-initiative development; and that supervisors could cultivate self-efficacy through the acquisition of skills and professional identity along with the development of values and beliefs. Recommendations for further research included using the Delphi method and participatory action research to build consensus statements and guidelines. Sharing these findings can inform positive social change by supporting supervisors as they prepare school counselor trainees for a successful career, in turn improving the social emotional development of their students in their local districts. [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.]