The use of teaching assistants (TAs) is widespread in many education systems, but the ways that TAs can support learning effectively are poorly understood. Much evidence indicates that most TA support has no, or negative, effects on pupil attainment. A small but growing body of evidence shows that structured TA intervention programmes can be effective. This paper reports the results of a large-scale randomised controlled trial of Catch Up® Numeracy, an intervention delivered one-to-one by TAs, compared to a control in which TAs provided matched-time numeracy support. The trial involved 1794 low-attaining pupils (aged 7-10) and 300 TAs from 150 English primary schools. Pupils in the intervention group showed no gains in attainment compared to the matched-time control, but there was some evidence to suggest a positive impact in pupils' attitudes. The implications for future TA interventions to address low attainment in numeracy are discussed.