Many children who would not be identified as having special educational needs are low-attaining in mathematics, which often has a severe impact on their progress at school and their successes in later life. This paper describes Catch Up Numeracy, a non-intensive targeted intervention for children who are low-attaining in mathematics, which is delivered by classroom assistants in only thirty minutes per week. Data for 440 children, including controls, show that children receiving Catch Up Numeracy intervention attained average gains more than twice that expected of typically attaining children, and more than twice that attained by children receiving non-targeted mathematics support. The evidence suggests that Catch Up Numeracy is effective for children who are low-attaining in mathematics. More generally, it supports the view that many children's arithmetical difficulties are highly susceptible to intervention, and that the intervention does not need to be intensive or delivered by highly-trained teachers to be effective.