For trainees to participate meaningfully in workplace-based assessment (WBA), they must have trust in their assessor. However, the trainee's dependent position complicates such trust. Understanding how power and trust influence WBAs may help us make them more effective learning opportunities. We conducted semi-structured interviews with 17 postgraduate anaesthesia trainees across Australia and New Zealand. Sensitised by notions of power, we used constructivist grounded theory methodology to examine trainees' experiences with trusting their supervisors in WBAs. In our trainee accounts, we found that supervisors held significant power to mediate access to learning opportunities and influence trainee progress in training. All episodes where supervisors could observe trainees, from simply working together to formal WBAs, were seen to generate assessment information with potential consequences. In response, trainees actively acquiesced to a deferential role, which helped them access desirable expertise and minimise the risk of reputational harm. Trainees granted trust based on how they anticipated a supervisor would use the power inherent in their role. Trainees learned to ration exposure of their authentic practice to supervisors in proportion to their trust in them. Trainees were more trusting and open to learning when supervisors used their power for the trainee's benefit and avoided WBAs with supervisors they perceived as less trustworthy. If assessment for learning is to flourish, then the trainee–supervisor power dynamic must evolve. Enhancing supervisor behaviour through reflection and professional development to better reward trainee trust would invite more trainee participation in assessment for learning. Modifying the assessment system design to nudge the power balance towards the trainee may also help. Modifications could include designated formative and summative assessments or empowering trainees to select which assessments count towards progress decisions. Attending to power and trust in WBA may stimulate progress towards the previously aspirational goal of assessment for learning in the workplace.