Drawing on the methodology of critical incidents, this study sets out to explore the perspectives that 20 experienced in-service mentors in Israel adopt towards critical incidents in their work, as they account for how they reason about and act upon these incidents. Mentors' stories of critical incidents shed light on the complex nature of mentors' professional expertise, reflected in the finding that experienced mentors' reasoning and behaviour constantly fluctuates between a novice and an expert stage, depending upon the nature of the situation and the type of mentor-mentee interaction that the mentor is confronted with. The study supports the more dynamic, discontinuous and interactionist view of the acquisition of expertise, highlighting the regressions and progressions that play out when experienced professionals take up an additional role, such as in the passage from teaching to mentoring.