The objectives of the study were to identify the professional issues that teachers perceived as important in their commitment to a health promotion (HP) programme launched in their schools and to understand their perceptions of the impact of the programme on themselves as professionals, individuals, on students, on school staff and on the relationship with students' families. A mixed methods design was used. An anonymous questionnaire was distributed to 54 participating teachers exploring their practices and perceptions of the programme and 26 semi-structured interviews were conducted which examined their professional commitment to the programme. The main factors that teachers identified as shaping their commitment were (1) their perceptions of the programme, specifically, its congruence with their own role and practice and also their perceived impact of the programme upon whole staff relations and (2) the specific school environment including school organization, quality of the relationships with parents and student behaviour. If HP programmes are to be successfully developed in schools, it is necessary to anchor them within the schools' mission. HP programmes need to make sense to teachers' educational perspectives. They need to be responsive to school needs. They also need to be cognisant of the internal tensions that programme implementation can engender among the whole staff, some of whom may be committed to HP in their school, while others, may not value HP in the same way. Therefore, implementation processes that are respectful of the challenges schools encounter and of the differing ontological perspectives that teachers may hold with regard to HP is important.