Supervision is an essential component in counselor education and training. Supervision helps Counselors-in-Training (CITs) evolve to fully functional counseling professionals. CITs start to receive supervision in practicum and internship. Throughout this experience, both clinical mental health and school counseling students work as a professional counselor with real clients, and their clinical practice will be supervised by a supervisor. After graduation, the graduates of clinical mental health counseling will receive years of post-graduate supervision, as required by state licensing boards, in order to obtain licensure. While the importance of supervision is irreplaceable in counseling, the factors that contribute to successful supervision outcomes, such as CIT self-efficacy is a prominent topic in the research of clinical supervision. Supervisor competency plays a vital role in the supervision relationship, which is associated closely with the success of supervision and the development of CITs' counseling self-efficacy. Self-efficacy comes from Bandura's social learning theory and has become an important measure for learning or counseling outcomes. CITs' training progress in supervision can be assessed through their counseling self-efficacy. Therefore, this study was designed to explore the relationship between supervisor competency and the counseling self-efficacy of CITs. An instrument, Psychotherapy Supervision Development Survey-Supervisee (PSDS-S), was revised to collect CITs' perceived supervisor competency and measure how much this perception contributes to their development of counseling self-efficacy. The outcomes revealed that the direct client contact hours and perceived supervisor competency contributed to 21.2% of the variance in counseling self-efficacy. The results indicate that CITs' counseling self-efficacy can be influenced by their perceived supervisor competency of their clinical supervisor. Counselor educators are informed by the results of the study, and it is critical for counselor educators to re-consider strategies of collaborating with competent site supervisors to ensure ideal development of CITs' counseling self-efficacy. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]