For over a decade, school-based mentoring has been employed as a key strategy in initial teacher training programmes. The diversity apparent in training settings as well as trainees' backgrounds raises questions in relation to the mentoring practice and the knowledge base that underpins it. By adopting a phenomenological, social constructivist approach the study aims to investigate mentors' perceptions of the relationship between their professional practice and the knowledge that informs it. It seeks to construct a typology of mentors professional knowledge in relation to Shulman's (1987) model of teacher knowledge and the statutory standards framework for initial teacher training and induction in England, which serves as a point of reference for all those involved in the assessment of professional competence during initial training and induction. The findings of this study suggest that in their work with trainee teachers and newly qualified teachers the majority of mentors draw on their teacher knowledge without sufficiently taking into account the specific aspects of adult learners and the generic principles underpinning mentoring. (Contains 3 tables, and 2 figures.)