This study examines how interpretations of mentoring by trainee mentors (TMs) changed over the course of a mentor training programme, and how this contributed to the TMs' professional development. The context of the study was a mentor training programme for preschool teachers who mentor early childhood teacher students during their practicums. This article presents a thematic content analysis of qualitative narrative data gathered from the TMs' narrative writings on the mentor training programme (N = 36) and the TMs' contributions at one focus group interview (N = 5). The findings suggest that the TMs' interpretations produced two main themes. First, changes in the interpretations were recognized concerning the task of mentoring, learning, and the relationship of the mentor and the student. Secondly, the TMs gained in confidence and expertise. The TMs thus developed their professional identity as mentors. Mentor training prepared the TMs for the mentoring process.