Switch-on is an intensive, targeted literacy intervention that aims to improve the reading skills of pupils who are struggling with literacy. There are two versions of the intervention: Switch-on Reading and Switch-on Reading and Writing. Both involve specially trained Teaching Assistants (TAs) delivering a tailored programme of literacy support in daily 20-minute sessions over a ten-week period. A previous EEF-funded evaluation of Switch-on had shown signs of promise in raising reading outcomes for Year 7 pupils, hence a larger scale effectiveness evaluation was conducted. Each of the 184 participating schools were randomly assigned to receive either Switch-on Reading, Switch-on Reading and Writing, or continue their usual practices of supporting pupils with reading difficulties (called 'business as usual' control). Between two and four TAs in each school were trained to deliver the interventions by a team of trainers who were distinct from the developers. TAs received 1.5 days training for Switch-on Reading, and 2.5 days for Switch-on Reading and Writing. Each TA worked with between two and four Year 3 pupils who had not met age-related expectations in literacy at the end of Key Stage 1, and who did not have a high level of special needs. In total, 999 pupils were involved in the trial. The primary outcome measure was the Hodder Group Reading Test 2A. The evaluation was conducted over the 2015/2016 academic year. A process and implementation evaluation was carried out which included depth telephone interviews with the developers, trainers, and TAs, and six case studies incorporating the views of TAs, class teachers, and Switch-on co-ordinators in the school. Key conclusions include: (1) Participating children in schools delivering either version of Switch-on made no additional progress in reading compared to similarly struggling children in 'business as usual' control schools. The 4 padlock security rating means that we have high confidence that there was no difference, and that this was due to Switch-on and not affected by other factors; (2) A similar result was observed for children eligible for Free School Meals; (3) The secondary analysis suggests that Switch-on might have affected children who did not actually receive the intervention by changing the make-up of their class or the capacity of their TAs. These findings are tentative, but emphasise the importance of considering potential impacts on all children when using targeted interventions; (4) Overall, participating staff were positive about the intervention and accompanying training. Perceived outcomes for pupils included increased confidence, motivation and interest in reading, and improved reading and writing skills. Skills of participating TAs were also felt to have improved; and (5) Some schools reported modifications to the prescribed content, duration and format of Switch-on sessions. Closer monitoring by the developers could help to ensure greater fidelity in future. It was also reported that some class teachers had limited awareness of Switch-on. Clarifying the role of Switch-on as part of a broader, teacher-led literacy strategy could improve implementation.