The Mathematics Mastery programme is a whole-school approach to teaching mathematics that aims to raise attainment for all pupils and close the attainment gap between pupils from low income families and their peers. The programme aims to deepen pupils' conceptual understanding of key mathematical concepts. This clustered Randomised Controlled Trial (RCT) investigated whether implementing the Mathematics Mastery programme led to improvement in Year 7 pupils' maths test scores. It was intended that schools would also begin to use the programme in Year 8 in the second year of implementation, and continue until the approach was in place across the school. 44 schools from London and the South East participated in the trial, with a total sample of 5,938 pupils. Participating schools received training and resources to support the adoption of the programme, which was delivered by the education charity, Ark. The project was one of two evaluations of Mathematics Mastery funded by the Education Endowment Foundation (EEF). Key conclusions include: (1) On average, Year 7 pupils in schools adopting Mathematics Mastery made a small amount more progress than pupils in schools that did not. However, the effect detected was not statistically significant, meaning that it is not possible to rule out chance as an explanation; (2) There is no strong evidence that the approach had a greater impact on lower-attaining pupils than on higher-attaining pupils; (3) Combining the findings from this study and a second randomised controlled trial of Mathematics Mastery involving Year 1 pupils may strengthen the overall evidence for the approach; (4) Given the low per-pupil cost, Mathematics Mastery may represent a cost-effective change for schools to consider. However, teachers would need to resolve tensions related to differentiation to provide support for all groups of children; and (5) It would be worthwhile to track the medium- and long-term impact of the approach, to assess whether there is a cumulative effect to the approach and whether it has an impact on performance in high-stakes tests. [Tests were supplied by an external contractor (GL Assessment). For the Primary Evaluation Report, see ED581183. For the Overarching Summary Report, see ED581180.]