Background: For prospective teachers, the school-based learning component of their teacher education programme is a crucial, and often demanding, part of becoming a teacher. During this time, mentor-teachers work in close collaboration with student teachers, who are often teaching in an actual school setting for the first time. As the relationship between mentor-teacher and student teacher is pivotal to the quality of work-based learning and to supporting the professional development of prospective teachers, more needs to be understood about this complex dynamic. Purpose: Our study aimed to contribute to this area by investigating, from the viewpoint of mentor-teachers, how student teachers respond to the demands of work-based learning. It had a particular focus on mentor-teachers' perceptions of student teachers' emotional challenges associated with teaching and engagement. Methods: Data consisted of interviews with 22 experienced mentor-teachers from five different municipalities. Data were analysed qualitatively, using tools aligned with a constructivist grounded theory approach. Findings: Describing teaching as a complex practice, the mentor-teachers regarded proactive engagement in developing professionalism, and ability to make appropriately differentiated adjustments to their teaching as important criteria in their evaluation of student teacher progression. The mentor-teachers described student teachers' emotional challenges in relation to these and discussed support strategies. Conclusions: The findings of the study highlight the complex dynamic of work-based learning and the key role that mentor-teachers can play in supporting the development of prospective teachers entering the profession, including the navigation of emotionally challenging situations.