The small-scale research study reported in this paper aimed to explore the ways in which mentors in primary school settings approach the assessment of trainee teachers within the English Initial Teacher Education system. Consistency and accuracy of assessment is judged by Ofsted (Office for Standards in Education) as a key indicator of quality, and yet very little is understood about the ways in which assessment is enacted by mentors. The paper explores some of the tensions involved in mentor assessment of trainees: variable interpretations of criteria, the dichotomous role of the mentor and differing operating contexts and personal constructs of mentors. It is concluded that these all influence mentor decision making, and a ‘mentor assessment identity’ is theorised, based on mentors’ previous and current experiences, contexts and beliefs. Consistency in terms of the application of criterion-based assessment to trainees’ teaching is thus fundamentally critiqued, and a way forward is suggested based on qualitative formative feedback. It is thus suggested that ITE providers could move away from grading and focus instead on strengths and areas for development for trainee teachers.