Does teacher professional development make a difference? How do we know? While researchers and policy makers acknowledge that teacher professional development (PD) needs to be assessed and evaluated there is often little clarity as to how this can be achieved. Evaluation of teacher PD has been described as the weak link in the PD chain despite it being linked with improved PD experiences and pupil outcomes. This may be due to a lack of skills and tools to carry out such evaluations or indeed it may be linked to how PD is conceptualised. This article explores extant literature and models of evaluation, revealing gaps in existing evaluation frameworks resulting in the development of a provisional PD evaluation framework for use in a study which set out to formally evaluate the impact of a PD initiative on teachers’ professional learning in five urban primary disadvantaged schools in the Republic of Ireland. Following application within this study the framework was critiqued and revised resulting in a new PD Impact Evaluation Framework which has subsequently been further tested to add to its robustness and may support teachers, researchers and policy makers to carry out systematic and focused evaluations of teacher PD.