Objectives Postgraduate trainee selection is a high-stakes process. While many studies focused on selection methods and psychometrics, little is known about the influence of selectors' personal values and beliefs in the judgment and decision-making process. A better understanding of these factors is vital since selectors determine the future workforce. Methods We interviewed programme directors (PDs) from 11 specialties in one University Hospital. Thematic analysis was conducted with a combined approach of generic and in-vivo coding. Results PDs value excellence, 'fit' and personal characteristics. The content of these values are subject to personal interpretation and differ between PDs. PDs use various 'proxies' as alternative indicators of performance. They consider intuition, teamwork and autonomy important in judgement and decision-making. PDs find selection challenging and feel great accountability towards candidates and society. Conclusions Selectors criteria of judgement- and decision-making often remain implicit and focus on prior achievements and 'fit' with the current trainee-pool, possibly compromising the workforce's diversity. Implicit 'proxies' and intuitive decision-making may be an unwitting source of judgemental bias. 'Making the implicit explicit', by increasing awareness of personal values and beliefs and structuring the selection interview, may improve the quality of trainee selection.