Supervisors provide evaluations, act as professional gatekeepers, and serve a central function to psychotherapy training, yet little is known about the impact of power dynamics on the supervisory relationship and the psychotherapy trainee. The present study offers a closer examination of Bordin's (1983) concept of supervisory alliance, revealing a largely unappreciated emphasis on defusing tension associated with power. This study is the first to investigate how soft and hard power impact supervisory alliance, while also further validating the recently developed Power Dynamics in Supervision Scale (PDSS; Cook, McKibben & Wind, 2018). The Core Conflictual Relationship Theme (Luborsky & Crits-Christoph, 1998) enriched results with qualitative investigation of supervisors' response style. A diverse sample of N = 311 psychotherapy trainees and a subset of 20 matched pairs of trainees and supervisors completed an online self-report survey. A model involving hard power and positive response style by supervisors predicted 56% of variance in trainee-rated supervisory alliance, and a model involving soft power and negative response style by supervisors predicted 51% of variance in alliance. Supervisor use of soft power appeared to serve as a buffer to trainees' perceived negative response styles by supervisors. Whether supervisors explicitly established goals at the outset of supervision predicted 13% of supervisory alliance and 6% of perceived balance of power. Concurrent validity testing supported the construction of the PDSS. Far fewer matched pair supervisory dyads were recruited than aimed for, but in the matched pair subsample, small, non-significant associations were found between trainee and supervisor ratings of alliance, hard and soft power, and the PDSS. Implications of the results for deepening the quality of supervisor-trainee dyads are discussed. [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.]