Despite economic difficulties, the emphasis on and investment in teacher professional development (PD) across the world continues, as countries strive to improve educational standards to compete in a globalised knowledge economy. However, researchers have little evidence of its impact on teachers’ professional practice. While it is acknowledged that PD needs to be assessed and evaluated, there is little guidance as to how this might be achieved. Much focus is on short-term impact, with longer-term impact often ignored despite sustainability of practices being highlighted as critical for school improvement. This study set out to explore the impact of a collaborative PD initiative on teachers’ professional practice in five urban disadvantaged primary schools in the Republic of Ireland. A qualitative approach was used to explore short term and longer-term impact, along with factors that helped or hindered the development and sustainability of the PD practice. The literature review revealed gaps in existing frameworks for evaluation, resulting in the development of a ‘Professional Development Impact Evaluation Framework’ which is presented in the thesis. It demonstrates how the framework was both developed from extant literature and critiqued through application, and discusses its potential for evaluating the impact of a range of PD activities and answering the call for accountability in these straitened times. Findings revealed a PD legacy that resulted not only in practices being sustained, but demonstrating a PD multiplier, where the impact of the collaborative PD initiative extended beyond the initiative itself to include many changes, even at a cultural level. Given the significance of the PD multiplier, this study suggests that PD facilitators support such cultural changes on a larger scale in schools. A significant feature of change is the teacher as a change-agent, and this study proposes a number of typologies of teacher engagement which may have some implications for teacher PD. Impacting on these typologies were three key elements that contributed to teachers’ professional learning and which reflect a developing notion of agentic teacher professionalism: bottom-up approaches with top-down support; autonomy and professional trust; and collaborative practices and collective responsibility.