This qualitative study investigated the qualities of veterinary student performance that inform a supervisor's impression of their competency. Semi-structured interviews were conducted with a purposive sample of 15 supervisors from different veterinary subdisciplines, to elicit descriptions of excellent, weak and marginal students. Thematic analysis of transcriptions revealed 12 themes, of which engagement was frequently discussed and of stated importance, and trustworthiness was a differentiator of weak and marginal students from excellent students. Other themes were knowledge, application of knowledge, technical and animal handling skills, communication, social interaction, personal functioning, caring for animals, impact, prospects and the difficulty in judging competency. Patterns of association of themes were found, however themes were also used independently in unique combinations for most students described. The findings show the range of abilities, behaviours, attitudes and personal characteristics of students that are considered by supervisors and how these are weighted and balanced. The key contribution of engagement and trustworthiness to the overall impression aligns with research indicating their importance for success in clinical practice, as both contributors to competency and indicators of it. The findings may inform future design and investigation of workplace-based learning and in-training evaluation, as well as conceptions of veterinary competency.