This article argues that evaluation of professional development (PD) requires a focus on student learning and an evidential baseline to enable practitioners and school leaders to determine the impact of the PD in which they are engaged. Several models of evaluating PD are briefly considered and it is suggested that most of these enable programme developers or commissioners to evaluate the impact of what they offer. However, many still struggle with evaluating the impact of PD and are looking for practical yet rigorous ways to achieve this. This paper suggests how this can be achieved, thus enabling schools to know and demonstrate that PD has had an impact, whilst also offering a powerful method to raise both the quality of learning--for adults and students--and student outcomes. In so doing, an approach to impact evaluation is presented. The main findings of the Effective Practices in Continuing Professional Development project are drawn upon to illustrate how this approach to impact evaluation can be deployed. The key lessons learned in relation to effective PD and impact evaluation are outlined. The model outlined gives importance to establishing an evidential baseline and impact picture that supports both adult and student learning.