Chess in Primary Schools is a whole-school approach to teaching primary school children how to play chess. Children take 30 hours of chess lessons delivered by a tutor who is an experienced chess player, and the school is given the option to set up a chess club as a lunchtime or after-school activity. Chess classes are delivered during the school day and are expected to replace subjects such as music or PE. The intervention was evaluated using a two-armed randomised controlled trial. The trial took place over the 2013/2014 academic year and assessed the impact of one year of Chess in Primary Schools on the mathematics attainment of pupils in Year 5. It was an effectiveness trial, with the intervention tested under realistic conditions in a large number of schools. This study looks at whether the intervention had an impact on attainment one year after the intervention had ended in June 2015. One hundred schools across 11 local education authorities (LEAs) in England participated in the trial, a total of 4,009 pupils. A process evaluation was also carried out to answer questions about implementation and to help explain the findings of the trial. The programme was delivered by the education charity Chess in Schools and Communities (CSC). Key conclusions include: (1) There is no evidence that the intervention had a positive impact on mathematics attainment for the children in the trial, as measured by Key Stage 2 scores one year after the intervention ended. The same is true for science and reading; (2) There is no evidence that the intervention had a positive impact on Key Stage 2 scores for children eligible for free school meals (FSM); (3) Although a current school teacher is allocated to every chess class, it is desirable for the tutors themselves to have strong class management and teaching skills. Without these, it was difficult to ensure that all children were suitably engaged in the chess lessons; (4) For successful implementation, class teachers need to work closely with the tutor and actively contribute to the intervention. It was felt that classes were less effective if the teacher did not actively take part, or was present only at the beginning and end of the class; and (5) Half of the pupils who participated in the trial said that they liked the chess lessons a lot, and only 8% reported that they didn't like them. School teachers were very positive about the intervention and its impact on pupils' skills and behaviour.