This was a randomised controlled trial involving 150 schools. Schools were randomly allocated to either deliver Catch Up® Numeracy or be in an ‘active control’ comparison group delivering a broadly time equivalent amount of TA-led numeracy support sessions informed by the EEF’s ‘Making best use of teaching assistants’ guidance report. The implementation and process evaluation involved: surveys of TAs, school coordinators, and senior leaders; analysis of delivery data; and case studies of intervention and comparison schools. The intervention began in September 2016 and finished in July 2017. Impact was measured using the Progress Test in Maths by GL Assessment.