This study investigates the influence of a two-year professional development programme on primary teachers' attitudes towards primary science, their confidence and competence in teaching science, and pupils' attitudes towards school science. Unlike the traditional "one-size-fits all" model of professional development, the model developed for this study, carried out in Ireland, is ongoing and gives teachers opportunities to collaborate with their colleagues, sharing practices and knowledge, to reflect on their pedagogic practice, to focus on pupil learning and to be involved in decision-making. This article explores the relationship, if any, between teacher involvement in a sustained, collaborative professional development programme, their confidence and competence in teaching science and their classroom practice. Findings from this study have shown that the participants become substantially more confident and competent in teaching primary science. Most significantly, teachers have made dramatic changes to the way they teach science, resulting in their pupils becoming more positive and motivated to learn science.