Workplace-Based Assessment (WBA) plays a pivotal role in present-day competency-based medical curricula. Validity in WBA mainly depends on how stakeholders (e.g. clinical supervisors and learners) use the assessments—rather than on the intrinsic qualities of instruments and methods. Current research on assessment in clinical contexts seems to imply that variable behaviours during performance assessment of both assessors and learners may well reflect their respective beliefs and perspectives towards WBA. We therefore performed a Q methodological study to explore perspectives underlying stakeholders’ behaviours in WBA in a postgraduate medical training program. Five different perspectives on performance assessment were extracted: Agency, Mutuality, Objectivity, Adaptivity and Accountability. These perspectives reflect both differences and similarities in stakeholder perceptions and preferences regarding the utility of WBA. In comparing and contrasting the various perspectives, we identified two key areas of disagreement, specifically ‘the locus of regulation of learning’ (i.e., self-regulated versus externally regulated learning) and ‘the extent to which assessment should be standardised’ (i.e., tailored versus standardised assessment). Differing perspectives may variously affect stakeholders’ acceptance, use—and, consequently, the effectiveness—of assessment programmes. Continuous interaction between all stakeholders is essential to monitor, adapt and improve assessment practices and to stimulate the development of a shared mental model. Better understanding of underlying stakeholder perspectives could be an important step in bridging the gap between psychometric and socio-constructivist approaches in WBA.