To mark the ten year anniversary since the introduction of the free schools programme, New Schools Network (NSN) commissioned the National Foundation for Educational Research (NFER) to conduct an independent data-led investigation into what impact free schools have had since their introduction in 2010. The research examines the impact that free schools have had since 2010 through the lens of three broad research questions: (1) How do pupils in free schools perform in terms of attainment outcomes compared to their peers in other types of schools, at different key stages of education?; (2) Have free schools managed to establish themselves with families as a credible and popular choice for educating their children?; and (3) How do the demographics and supply dynamics of the teacher workforce in free schools compare to other schools? This report focuses on mainstream free schools only (that is, special and alternative provision free school have been excluded). University Technical Colleges and studio schools are also excluded. Findings suggest that there is room for free schools to improve their retention of both classroom teachers and senior leaders, which is lower than in other schools. Further research is needed to establish what is driving lower retention in new schools such that government and school leadership can better support teachers, and thereby retain more in the school and the system. [This research was commissioned by New Schools Network.]