This article examines dilemmas in measuring the impact of mentoring on teacher trainees and on their learners in the lifelong learning sector (LLS). In this small-scale research project, five mentors and five mentees were asked how they might evaluate the impact that mentoring might have, not just on trainees, but on their learners. The research participants, who were from different London colleges which all run a University accredited initial teacher education course, were asked: how might they measure mentoring impact? To what extent might these forms of evaluation be considered valid and reliable? The implementation of formal mentoring for teacher trainees in the lifelong learning sector has increased the need for systematic evaluation of mentoring schemes by universities and colleges in initial teacher education. The mentors' and mentees' suggestions for evaluating the impact of mentoring comprised quantitative and qualitative methods and also illustrated the significant challenges to evaluating, with any precision, the benefits of mentoring in hard statistical terms.