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 summary is based on two randomised controlled trials of Mathematics Mastery funded by the Education Endowment Foundation. The first evaluation assessed the impact of the programme on pupils in Year 1. 83 schools from London and the South East participated in the trial, with a total sample of 4,176 pupils. The second assessed the impact of the programme on pupils in Year 7. 44 schools from London and the South East participated in the trial, with a total sample of 5,938 pupils. Both evaluations assessed the programme's impact in its first year of adoption. In subsequent years it was intended that schools would begin to use the approach with older year groups until it was in place across all year groups. The education charity, Ark, provided participating schools with training and resources to support the adoption of the programme. Key conclusions include: (1) This summary is based on findings from two randomised controlled trials conducted in English schools between 2011 and 2014; (2) On average, pupils in schools adopting Mathematics Mastery made a small amount more progress than pupils in schools that did not. The effect detected was statistically significant, which means that it is likely that that improvement was caused by the programme; (3) It is unclear whether the programme had a different impact on pupils eligible for free school meals, or on pupils with higher or lower attainment; (4) Given the low per-pupil cost, Mathematics Mastery may represent a cost-effective change for schools to consider; and (5) The evaluations assessed the impact of the programme in its first year of adoption. It would be worthwhile to track the medium and long-term impact of the approach. [For the Primary Evaluation Report, see ED581183. For the Secondary Evaluation Report, see ED581187.]