Purpose Supervision is typically mandatory for therapists in training and plays an important role in their professional development. A number of qualitative studies have considered specific aspects of supervision. This systematic review aimed to synthesize these studies' findings and explore the experience and impact of supervision for trainee therapists. Methods A systematic search of the literature was conducted, and inclusion/exclusion criteria were applied. This led to a sample of 15 qualitative studies, with which a meta‐synthesis was conducted. Results The meta‐synthesis led to four key concepts: supervision as a learning opportunity, the supervisory relationship, power in supervision and the impact of supervision. These themes explored helpful and unhelpful aspects of supervision, including some concerns regarding the evaluation of supervision. Conclusions Supervision can effectively support trainee therapists in their personal and professional development. However, it can also lead to feelings of distress and self‐doubt. Supervisors need to consider the power differential within supervision and attend to different factors within the supervisory relationship. Copyright © 2015 John Wiley & Sons, Ltd. Key Practitioner Message Supervision can encourage personal and professional development, but it can also have a detrimental impact on trainee therapists' well‐being, and consequently their clinical work and clients' experiences. Supervisees may not disclose unhelpful events or impacts from supervision, for fear of negative evaluation. Evaluation of supervisors should be facilitated and encouraged, to maintain good practice.