The Incredible Years Teacher Classroom Management (TCM) programme is an evidence-based training with documented positive effects on teachers' classroom management strategies and pupil mental health. However, programme effectiveness alone does not ensure sustainability. This study explored teachers' views on the sustainability of the TCM programme. One year after attending the TCM training, 25 UK primary schoolteachers participated in three focus groups and ten individual interviews. Data were thematically analysed using the Framework Method. One year after the training, teachers were still using TCM strategies, adapting their use to their new cohort of pupils and still reported positive effects on children, teacher-parent relationships, and themselves. However, some school-level and educational policy changes hindered their intended use of TCM strategies. Teachers had shared the TCM strategies with their colleagues, with mixed responses from the latter. To ensure the sustainability of the programme, teachers endorsed refresher sessions and a whole-school training approach.