Self-regulated learning is essential for professional development and lifelong learning. As self-regulated learning has many inaccuracies, the need to support self-regulated learning has been recommended. Supervisors can provide such support. In a prior study trainees reported on the variation in received supervisor support. This study aims at exploring supervisors' perspectives. The aim is to explore how supervisors experience self-regulated learning of postgraduate general practitioners (GP) trainees and their role in this, and what helps and hinders them in supervising. In a qualitative study using a phenomenological approach, we interviewed 20 supervisors of first- and third-year postgraduate GP trainees. Supervisors recognised trainee activity in self-regulated learning and adapted their coaching style to trainee needs, occasionally causing conflicting emotions. Supervisors' beliefs regarding their role, trainees' role and the usefulness of educational interventions influenced their support. Supervisors experienced a relation between patient safety, self-regulated learning and trainee capability to learn. Supervisor training was helpful to exchange experience and obtain advice. Supervisors found colleagues helpful in sharing supervision tasks or in calibrating judgments of trainees. Busy practice occasionally hindered the supervisory process. In conclusion, supervisors adapt their coaching to trainees' self-regulated learning, sometimes causing conflicting emotions. Patient safety and entrustment are key aspects of the supervisory process. Supervisors' beliefs about their role and trainees' role influence their support. Supervisor training is important to increase awareness of these beliefs and the influence on their behaviour, and to improve the use of educational instruments. The results align with findings from other (medical) education, thereby illustrating its relevance.