Objectives: In 2010 the Royal College of Radiologists (RCR) introduced workplace based assessment (WPBA) into clinical radiology training. The study purposes were firstly to investigate the implementation of WPBA by examining its day-today realisation in practice and to relate these findings to the guidance provided by the RCR. The second aim was to explore where the usage of WPBA differed from the RCR's declared intention and to examine the reasons for this deviation. Methods: Semi-structured interviews were undertaken with 20 radiologists (12 trainers, 8 trainees) to explore their opinions on the implementation and usage of WPBA in radiology. The interview data were analysed using thematic analysis. Results: The themes derived from the analysis were amending the assessments' purpose, altering WPBA's usage depending upon trainees' seniority or time in an attachment, amending the recording of assessments, and assessing trainees without their knowledge. In terms of amending the assessments' purpose, all interviewees thought that WPBA had a formative ethos but some assessors felt it could be used summatively in certain circumstances. The day-today realisation of WPBA varied, and although this might be seen as inevitable due to differing circumstances, there was evidence of some trainers consciously amending the process. Conclusions: There was variation in the realisation of WPBA beyond that caused by their timing or context related to conscious adaptation of the process by trainers. Although such diversity of practice may or may not have impacted adversely upon the process, there is potential to introduce unfairness into WPBA and thereby disadvantage trainees.