Background: The Incredible Years® (IY) Teacher Classroom Management (TCM) programme may be an effective way to reduce teacher stress levels, improve child behaviour, and promote positive socio-emotional development. However, few studies have considered what teachers think of the course and how it might work. Aims: In this paper, we examine teachers' perceptions of the impact of the TCM programme and how it might work in the classroom. Sample(s): Forty-four UK primary school teachers who attended the TCM programme as part of the STARS trial (Ford "et al.," 2018, "Psychol. Med.," 49, 828). Methods: Focus groups and interviews were held with teachers two months after completing the TCM programme. Thematic analysis (Braun & Clarke, 2006, "Qual. Res. Psychol.," 3, 77) was employed to explore the subsequent data. Results: Three main themes were identified: impact on the teacher; on children; and on parent-teacher relationships. Impact on the teacher included a positive change in their ethos. Teachers reported being more able to see things from the child's perspective; placing a greater focus on building positive relationships; thinking before responding; feeling calmer, more confident, and in control; and employing strategies to create positive interactions with children. Teachers felt this had had a positive impact on their pupils' development and relationships with parents. Feedback on whether or not TCM was effective in tackling particularly challenging behaviour was more mixed. Conclusions: Our findings suggest that teachers experience the TCM programme as beneficial. This is discussed alongside other qualitative and quantitative studies in this field.