Challenge the Gap (CtG) is a school collaboration programme designed by Challenge Partners that aims to break the link between disadvantage and attainment. The main components of CtG are: (1) after-school workshops drawing on published research and evidenced practice; (2) focused in-school interventions with a selected cohort of disadvantaged pupils; (3) cross-school collaboration and practice development; and (4) practical tools and resources for use in schools and additional online materials. This project evaluated the 2012 version of the CtG programme. An initial pilot study began in late 2012 and focused on the feasibility of the approach with a group of 30 schools. The next phase of the evaluation involved 104 primary and secondary schools (21,041 pupils), which were funded by the Education Endowment Foundation (EEF) to take part in the programme between 2013 and 2014, and then given the option to pay for a further year between 2014 and 2015, taken up by 49 schools. The evaluation assessed the impact on all participating schools, using 2015 Key Stage 2 or Key Stage 4 results. The main evaluation was a matched-controlled efficacy trial. Process evaluation data was collected through surveys, school visits, interviews, and monitoring postings on the CtG website. The project found no evidence that CtG increased average attainment for either primary or secondary school pupils, overall. The security of the primary school results is low to moderate, and the security for the secondary school results is low. The findings are different for children eligible for free school meals. Free school meals (FSM)-eligible children in CtG primary schools made 2 months' additional progress compared to similar children in other schools, while FSM-eligible children in CtG secondary schools made 2 months' fewer progress compared to similar children in other schools. Teachers and non-teaching staff from participating schools were extremely positive about the involvement of their schools in the CtG programme and valued both the professional development opportunities it provided and the opportunity to collaborate with partner schools. CtG is a flexible programme that allows lead schools to share best practice. There was large variation between the strategies that were adopted by the schools in the programme.