Mathematics anxiety is a widespread problem that inhibits learning, despite it being preventable and manageable. This thesis reports a study which developed an intervention to support learners directly in the recognition and management of their mathematics anxiety. The research was conducted in two phases. Firstly, the existence of a substantial level of mathematics anxiety was established in eleven-year-old pupils in a Midlands comprehensive school, through an adapted mathematics anxiety scale. Secondly, pupils from this group with the highest levels of mathematics anxiety were offered the opportunity to take part in an intervention, and 13 pupils agreed. Three Design-Based Research cycles generated 32 clinical interviews which were delivered, recorded and analysed. The thesis supports the literature in that mathematics anxiety is negatively correlated to mathematical achievement, and more common in female learners than male learners. Qualitative analysis of the intervention found it to be efficacious. The thesis develops the Growth Zone Model introduced by Johnston-Wilder and Lee (2010) through the application of psychological strategies to reduce general anxiety and introduces a dynamic view of the model that is based in learner autonomy. Implications for future research and practical applications in school settings are discussed.